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Kernel panic with Jool v3.5.6 (kernel BUG at .../skbuff.h:1826) #247

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toreanderson opened this Issue Jun 12, 2017 · 15 comments

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toreanderson commented Jun 12, 2017

Within 24 hours of each other two of our SIIT-DC BRs running SIIT Jool v3.5.2 on Ubuntu 14.04.5 with kernel 4.4.0-51-generic panicked in a very similar fashion. They had both been running stable for over half a year, so the incidents are highly suspect. Possibly another «packet of death» issue akin to issue #232?

The only information I have about it is the traces that were output to the serial console as they crashed:

Trace from SIIT-DC BR 1
Trace from SIIT-DC BR 2

Tore

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Fun times ahead. ^^"

Thanks. It's pretty safe to assume that it crashed here. The line is saying "throw a panic if len is less than data_len". The fact that data_len was nonzero (as both fields are unsigned) implies that the packet was paged. Whether a packet is paged or not depends on random factors, which unfortunately means that this is not likely going to be as deterministic as 242. But at least the bug can be traced back to very few packet fields.

When did you update to 3.5.2?

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ydahhrk commented Jun 12, 2017

Fun times ahead. ^^"

Thanks. It's pretty safe to assume that it crashed here. The line is saying "throw a panic if len is less than data_len". The fact that data_len was nonzero (as both fields are unsigned) implies that the packet was paged. Whether a packet is paged or not depends on random factors, which unfortunately means that this is not likely going to be as deterministic as 242. But at least the bug can be traced back to very few packet fields.

When did you update to 3.5.2?

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toreanderson Jun 13, 2017

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I upgraded on the 6th of December 2016 (to fix #232). Both servers had been running stable since then.

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toreanderson commented Jun 13, 2017

I upgraded on the 6th of December 2016 (to fix #232). Both servers had been running stable since then.

@ydahhrk ydahhrk added this to the 3.5.4 milestone Jun 14, 2017

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Sorry; I'm still completely empty-handed. I didn't find anything strange by reviewing the code, and the new unit test reports nothing but happiness.

I'm wondering if you might be willing to run a custom version of 3.5.2 that would, instead of crashing, dump a bunch of additional information in the log and drop the packet. The reason why I'm hesitant to commit this enhancement to 3.5.4 is because the changes needed to pull this off are kernel-aware, and might therefore introduce new issues in other kernels.

(I'm probably being a crybaby. This kernel code hasn't changed at all for many years, and likely won't either for a long time...)

Did you update Jool to 3.5.3, and the kernel to the latest generic after the crash? If so, would you rather branch these changes off 3.5.3 instead of 3.5.2?

Aside:

When you say "within 24 hours of each other", could this by any chance be roughly the same amount of time that separated their activation? ie. Did you start the first BR, and then after 24 hours start the second one?

Are you using the atomic configuration feature?

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ydahhrk commented Jun 26, 2017

Sorry; I'm still completely empty-handed. I didn't find anything strange by reviewing the code, and the new unit test reports nothing but happiness.

I'm wondering if you might be willing to run a custom version of 3.5.2 that would, instead of crashing, dump a bunch of additional information in the log and drop the packet. The reason why I'm hesitant to commit this enhancement to 3.5.4 is because the changes needed to pull this off are kernel-aware, and might therefore introduce new issues in other kernels.

(I'm probably being a crybaby. This kernel code hasn't changed at all for many years, and likely won't either for a long time...)

Did you update Jool to 3.5.3, and the kernel to the latest generic after the crash? If so, would you rather branch these changes off 3.5.3 instead of 3.5.2?

Aside:

When you say "within 24 hours of each other", could this by any chance be roughly the same amount of time that separated their activation? ie. Did you start the first BR, and then after 24 hours start the second one?

Are you using the atomic configuration feature?

ydahhrk added a commit that referenced this issue Jun 26, 2017

Add unit tests for pagination
Still can't find the #247 bug.
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I'd be happy to try running a version with extra debugging. Just give me a git revision to check out and I'll upgrade in my next available maintenance window.

I did not upgrade Jool following the crash (still running 3.5.2), but the kernel was upgraded to 4.4.0-79-generic after the crash. It will be upgraded to 4.4.0-81-generic (or anything newer that might come out in the mean time) on the next boot, unless you'd rather I didn't.

On December 6th the two nodes were booted five minutes after one another, they did not have the exact same uptime when crashing. To be specific, node A was booted on 2016-12-06 01:24 while node B was booted 01:29. Then node B was first to crash on 2017-06-10 16:52, followed by node A on 2017-06-11 00:15.

For what it's worth there's also a third SIIT-DC BR in my network, running virtualised in KVM. This one was also rebooted on December 6th (01:11) in order to upgrade to 4.4.0-51-generic and Jool 3.5.2. Same versions and same configuration. It has not crashed yet.

Considering that I was the one to initially suggest it, I am a little bit embarrassed to say that I still haven't gotten around to updating my automation scripts to use the atomic configuration feature...

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toreanderson commented Jun 27, 2017

I'd be happy to try running a version with extra debugging. Just give me a git revision to check out and I'll upgrade in my next available maintenance window.

I did not upgrade Jool following the crash (still running 3.5.2), but the kernel was upgraded to 4.4.0-79-generic after the crash. It will be upgraded to 4.4.0-81-generic (or anything newer that might come out in the mean time) on the next boot, unless you'd rather I didn't.

On December 6th the two nodes were booted five minutes after one another, they did not have the exact same uptime when crashing. To be specific, node A was booted on 2016-12-06 01:24 while node B was booted 01:29. Then node B was first to crash on 2017-06-10 16:52, followed by node A on 2017-06-11 00:15.

For what it's worth there's also a third SIIT-DC BR in my network, running virtualised in KVM. This one was also rebooted on December 6th (01:11) in order to upgrade to 4.4.0-51-generic and Jool 3.5.2. Same versions and same configuration. It has not crashed yet.

Considering that I was the one to initially suggest it, I am a little bit embarrassed to say that I still haven't gotten around to updating my automation scripts to use the atomic configuration feature...

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Thanks. Working on the custom build right now. I still need to test it, so I wouldn't recommend you to download it yet.

I did not upgrade Jool following the crash (still running 3.5.2), but the kernel was upgraded to 4.4.0-79-generic after the crash. It will be upgraded to 4.4.0-81-generic (or anything newer that might come out in the mean time) on the next boot, unless you'd rather I didn't.

:/

I can't rule out the possibility that this might be a kernel bug (which at this point feels likely), in which case this update might prevent us from seeing the crash again. But I feel that I can't ask you to downgrade to 4.4.0-51-generic since that could net you unnecessary security issues.

Incidentally, is this bug preventing you from updating Jool to 3.5.3? Or do you have some other reason to stick to 3.5.2? Because if you updated the kernel anyway, we could just move over to the other extreme and just try to debug in the latest everything.

Considering that I was the one to initially suggest it, I am a little bit embarrassed to say that I still haven't gotten around to updating my automation scripts to use the atomic configuration feature...

Ok, no problem. That simplifies this.

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ydahhrk commented Jun 27, 2017

Thanks. Working on the custom build right now. I still need to test it, so I wouldn't recommend you to download it yet.

I did not upgrade Jool following the crash (still running 3.5.2), but the kernel was upgraded to 4.4.0-79-generic after the crash. It will be upgraded to 4.4.0-81-generic (or anything newer that might come out in the mean time) on the next boot, unless you'd rather I didn't.

:/

I can't rule out the possibility that this might be a kernel bug (which at this point feels likely), in which case this update might prevent us from seeing the crash again. But I feel that I can't ask you to downgrade to 4.4.0-51-generic since that could net you unnecessary security issues.

Incidentally, is this bug preventing you from updating Jool to 3.5.3? Or do you have some other reason to stick to 3.5.2? Because if you updated the kernel anyway, we could just move over to the other extreme and just try to debug in the latest everything.

Considering that I was the one to initially suggest it, I am a little bit embarrassed to say that I still haven't gotten around to updating my automation scripts to use the atomic configuration feature...

Ok, no problem. That simplifies this.

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On December 6th the two nodes were booted five minutes after one another, they did not have the exact same uptime when crashing. To be specific, node A was booted on 2016-12-06 01:24 while node B was booted 01:29. Then node B was first to crash on 2017-06-10 16:52, followed by node A on 2017-06-11 00:15.

For what it's worth there's also a third SIIT-DC BR in my network, running virtualised in KVM. This one was also rebooted on December 6th (01:11) in order to upgrade to 4.4.0-51-generic and Jool 3.5.2. Same versions and same configuration. It has not crashed yet.

Ok, thanks.

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ydahhrk commented Jun 27, 2017

On December 6th the two nodes were booted five minutes after one another, they did not have the exact same uptime when crashing. To be specific, node A was booted on 2016-12-06 01:24 while node B was booted 01:29. Then node B was first to crash on 2017-06-10 16:52, followed by node A on 2017-06-11 00:15.

For what it's worth there's also a third SIIT-DC BR in my network, running virtualised in KVM. This one was also rebooted on December 6th (01:11) in order to upgrade to 4.4.0-51-generic and Jool 3.5.2. Same versions and same configuration. It has not crashed yet.

Ok, thanks.

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toreanderson Jun 28, 2017

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I don't recall any security fixes in the kernel since -51-generic that would be applicable to these nodes (no local users or network-exposed services), so I can downgrade if you want (provided I can still find the deb packages for -51-generic somewhere).

The reason why I haven't upgraded to 3.5.3 is simply that the changes since 3.5.2 didn't seem relevant to me. Unlike Jool (which is tied to a specific git-rev), the kernel is updated automatically on-disk by the apt packaging system, so when the node crashed and was rebooted it activated the most recent version - it wasn't a conscious decision to update the kernel but not Jool, in case I gave you that impression.

In any case, I'll run whatever versions you'd want me to. Just let me know what you prefer.

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toreanderson commented Jun 28, 2017

I don't recall any security fixes in the kernel since -51-generic that would be applicable to these nodes (no local users or network-exposed services), so I can downgrade if you want (provided I can still find the deb packages for -51-generic somewhere).

The reason why I haven't upgraded to 3.5.3 is simply that the changes since 3.5.2 didn't seem relevant to me. Unlike Jool (which is tied to a specific git-rev), the kernel is updated automatically on-disk by the apt packaging system, so when the node crashed and was rebooted it activated the most recent version - it wasn't a conscious decision to update the kernel but not Jool, in case I gave you that impression.

In any case, I'll run whatever versions you'd want me to. Just let me know what you prefer.

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Okay, that's perfect.

(You sure you need to redownload the kernel? My Ubuntu Server doesn't uninstall old kernels by default.)

I just finished testing it. The commit is the same: 21b97b1.

Please, go back to 4.4.0-51-generic to maximize the probability of getting the error again, and look out for output that looks somewhat like this:

$ dmesg
[10131.934230] Bug #247 happened!
[10131.934834] Page size: 4096
[10131.935386] Page shift: 12
[10131.935962] Inner packet l4-protocol: 6
[10131.936577] initial len: 1120
[10131.937140] initial data_len: 1000
[10131.937695] initial nr_frags: 1
[10131.938241]     initial frag 0: 1000
[10131.938795] mid len: 1120
[10131.939342] mid data_len: 1000
[10131.939886] mid nr_frags: 1
[10131.940438]     mid frag 0: 1000
[10131.940983] current len: 999
[10131.941515] current data_len: 1000
[10131.942031] current nr_frags: 1
[10131.942529]     current frag 0: 1000
[10131.943030] Dropping packet.

It should not crash. The expression "Bug #247 happened!" is constant. This might help for grepping. "Dropping packet" is also constant.

And guess we'll have to wait another several months.

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ydahhrk commented Jun 29, 2017

Okay, that's perfect.

(You sure you need to redownload the kernel? My Ubuntu Server doesn't uninstall old kernels by default.)

I just finished testing it. The commit is the same: 21b97b1.

Please, go back to 4.4.0-51-generic to maximize the probability of getting the error again, and look out for output that looks somewhat like this:

$ dmesg
[10131.934230] Bug #247 happened!
[10131.934834] Page size: 4096
[10131.935386] Page shift: 12
[10131.935962] Inner packet l4-protocol: 6
[10131.936577] initial len: 1120
[10131.937140] initial data_len: 1000
[10131.937695] initial nr_frags: 1
[10131.938241]     initial frag 0: 1000
[10131.938795] mid len: 1120
[10131.939342] mid data_len: 1000
[10131.939886] mid nr_frags: 1
[10131.940438]     mid frag 0: 1000
[10131.940983] current len: 999
[10131.941515] current data_len: 1000
[10131.942031] current nr_frags: 1
[10131.942529]     current frag 0: 1000
[10131.943030] Dropping packet.

It should not crash. The expression "Bug #247 happened!" is constant. This might help for grepping. "Dropping packet" is also constant.

And guess we'll have to wait another several months.

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All right. Now both the BRs that crashed earlier are running Jool 21b97b1 on 4.4.0-51-generic. Then we wait...

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toreanderson commented Jun 30, 2017

All right. Now both the BRs that crashed earlier are running Jool 21b97b1 on 4.4.0-51-generic. Then we wait...

@ydahhrk ydahhrk removed this from the 3.5.4 milestone Jul 17, 2017

@ydahhrk ydahhrk added the I'm Stuck label Jul 20, 2017

ydahhrk added a commit that referenced this issue Jul 20, 2017

Merge branch 'issue247-custombuild'
It seems silly to only keep these validations in an obscure
previous branch. If the #247 crash happens to blow up in the latest
master, I will have only myself to blame for not getting the output
I need.

So I'm committing these debugging tweaks for posterity. Will revert
once #247 is fixed.

I just checked; every supported kernel so far will crash if this
condition is met, so this should not introduce any issues. (At
least up till kernel 4.13-rc1)
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To clarify:

To make sure that this bug does not affect more people, I have merged these tweaks (validation, debugging messages and packet drop) on the latest master. Hopefully, this will improve the probability of someone else reporting the output as well.

If you want to download Jool 3.5.4, don't let me stop you.

Downgrading to non-critical because it shouldn't crash anymore.

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ydahhrk commented Jul 27, 2017

To clarify:

To make sure that this bug does not affect more people, I have merged these tweaks (validation, debugging messages and packet drop) on the latest master. Hopefully, this will improve the probability of someone else reporting the output as well.

If you want to download Jool 3.5.4, don't let me stop you.

Downgrading to non-critical because it shouldn't crash anymore.

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New crash last night with Jool v3.5.6. The events seem very similar as the one originally reported last summer. This time, the line it complains about is skbuff.h:1826, so off by one, but I'm guessing that's simply because the kernel had been upgraded (from 4.4.0-51-generic to 4.4.0-116-generic).

My two physical boxes crashed within a minute of each other, while my third virtual one stayed up and saved the day. The three nodes are anycasted, so if there was some sort of incoming stream of «packets of death» from somewhere, it'd get rerouted from node to node as each individual one crashed, causing a domino effect. If that was the case, though, it would appear that the virtual instance is immune.

Here are the traces that appeared on the serial console as they crashed:

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toreanderson commented Apr 20, 2018

New crash last night with Jool v3.5.6. The events seem very similar as the one originally reported last summer. This time, the line it complains about is skbuff.h:1826, so off by one, but I'm guessing that's simply because the kernel had been upgraded (from 4.4.0-51-generic to 4.4.0-116-generic).

My two physical boxes crashed within a minute of each other, while my third virtual one stayed up and saved the day. The three nodes are anycasted, so if there was some sort of incoming stream of «packets of death» from somewhere, it'd get rerouted from node to node as each individual one crashed, causing a domino effect. If that was the case, though, it would appear that the virtual instance is immune.

Here are the traces that appeared on the serial console as they crashed:

@toreanderson toreanderson changed the title from Kernel panic with Jool v3.5.2 (kernel BUG at .../skbuff.h:1827) to Kernel panic with Jool v3.5.6 (kernel BUG at .../skbuff.h:1826) Apr 20, 2018

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NOOOOOOOOOOOOOOOOOOOOO, I MESSED UP THE FAILURE DETECTION CONDITIONAL orz

(╯°□°)╯︵ ┻━┻ WHAT WAS I THINKIIIIIIIIIIIIIIIIING

All right, it's worth a fresh review anyway. I'm going to be looking into it again next week.

Update to the latest commit (68b19ea) so it doesn't crash violently anymore. Hopefully I got it right this time.

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ydahhrk commented Apr 20, 2018

NOOOOOOOOOOOOOOOOOOOOO, I MESSED UP THE FAILURE DETECTION CONDITIONAL orz

(╯°□°)╯︵ ┻━┻ WHAT WAS I THINKIIIIIIIIIIIIIIIIING

All right, it's worth a fresh review anyway. I'm going to be looking into it again next week.

Update to the latest commit (68b19ea) so it doesn't crash violently anymore. Hopefully I got it right this time.

ydahhrk added a commit that referenced this issue Apr 24, 2018

Issue #247 review
Haven't really found the bug, but I'm questioning some of the code
anyway. Uploading tweaks.

While computing the nexthdr of a given IPv6 packet, the code was
performing the following assignment:

	Local network header pointer = skb_network_header(skb) + local network header offset

(By "local" I mean the scope of the variable.)

This operation is reused for outer and inner packets, and strongly
requires and assumes that skb_network_offset(skb) is zero:

- For outer packets, it doesn't make sense to add anything to
  skb_network_header(skb) to get a pointer to the network header,
  and "local network header offset" is skb_network_offset(skb).
- For inner packets, "local network header offset" is an offset
  from skb->data, which means that skb_network_header(skb) needs
  to equal skb->data. (Hence, skb_network_offset(skb) = 0.)

Which is supposed to always the case, which is the reason why this
isn't really a bug, unless the kernel itself has another bug to
match.

I thought about putting a validation in place to ensure that the
network header offset is zero for all outer packets, but then I
realized that the whole reason why I was making this so convoluted
was because I didn't want the code to touch skb->data. (Instead
relying on available kernel API.) I noticed that there is at least
one function around (offset_to_ptr()) that does it anyway.
(And I don't think that there is a way around that unless the
packet.c module is completely rewritten.)

So the new code is

	Local network header pointer = skb->data + local network header offset

Which actually expresses the intent much more clearly.

The following are old assumptions that stand valid still:

- The skb_network_offset() offset is relative to skb->data.
- The skb_header_pointer() offset is relative to skb->data.

The following is no longer an assumption and Jool should continue
working fine if the kernel happens to break it:

- skb_network_offset(skb) during a packet's entrance to a Netfilter
  hook is zero.

So that's what this commit comes down to; I removed an potential
future bug opportunity and made the code clearer as a bonus.

ydahhrk added a commit that referenced this issue Apr 25, 2018

More issue #247 review
The code was assuming that IPv6 headers were located in the data
area of skbs. This is not necessarily true, particularly for
ICMP internal packets.

I can't think of a way this could have caused the bug, but I'm
still looking.

ydahhrk added a commit that referenced this issue Apr 26, 2018

More issue #247 review
The code wasn't catching NULL as a potential result from
skb_pull().

In all fairness, nobody does, really. Standard skb operations
usually crash the kernel on failure; I don't really understand why
this specific error condition yields NULL instead of BUG(). And no
other kernel citizens seem to care about it.

Also, this is obviously not the cause of #247; it would have to
crash later if this were the case.

@ydahhrk ydahhrk removed the I'm Stuck label Apr 27, 2018

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Okay, I managed to design a nuke packet that triggers an error similar to the one you got.

I think that publishing the specs of the packet would be a bad idea since no one, aside from attackers, would benefit from the information. But I think it's safe to say that it's a very, very forced packet. You can't just put it on the network and expect Jool to crash; the kernel also needs to append some impressive random miracles on its own. I wouldn't say that there is a bug in the kernel since the packet technically does not break any contracts, but damn that's some convoluted black magic there.

The bug has been fixed since commit 5c90ead.

I cannot but conjecture as to why you're getting the crashes so close to each other, which is why I'd like to leave the data collection and validations in place for at least another year, just in case.

With the release of Jool 3.5.7 I'm going to close this issue (if you don't close it earlier, that is), but feel free to reopen if the crash finds you again, as usual.

Thanks, Tore. I'm only 99% sure that we fixed the bug, but I'm glad we got that particular crack all patched up.

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ydahhrk commented Apr 27, 2018

Okay, I managed to design a nuke packet that triggers an error similar to the one you got.

I think that publishing the specs of the packet would be a bad idea since no one, aside from attackers, would benefit from the information. But I think it's safe to say that it's a very, very forced packet. You can't just put it on the network and expect Jool to crash; the kernel also needs to append some impressive random miracles on its own. I wouldn't say that there is a bug in the kernel since the packet technically does not break any contracts, but damn that's some convoluted black magic there.

The bug has been fixed since commit 5c90ead.

I cannot but conjecture as to why you're getting the crashes so close to each other, which is why I'd like to leave the data collection and validations in place for at least another year, just in case.

With the release of Jool 3.5.7 I'm going to close this issue (if you don't close it earlier, that is), but feel free to reopen if the crash finds you again, as usual.

Thanks, Tore. I'm only 99% sure that we fixed the bug, but I'm glad we got that particular crack all patched up.

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Great job! 👍

I'll upgrade to 3.5.7 at first opportunity. I'm quite okay with the details of the nuke packet not being made public, although I certainly am curious... Well, you have my e-mail address, if you're willing to share more details privately. ;-)

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toreanderson commented Apr 28, 2018

Great job! 👍

I'll upgrade to 3.5.7 at first opportunity. I'm quite okay with the details of the nuke packet not being made public, although I certainly am curious... Well, you have my e-mail address, if you're willing to share more details privately. ;-)

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I'll upgrade to 3.5.7 at first opportunity.

Ok.

Just to clarify: Remember that you don't need to wait for the release; it's just a formality so people can see an incrementing number and not install autoconf. The patches are already collapsed into master, and I run the entire test suite before committing to that.

Well, you have my e-mail address, if you're willing to share more details privately. ;-)

I think that's fine, but I will release 3.5.7 first.

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ydahhrk commented Apr 28, 2018

I'll upgrade to 3.5.7 at first opportunity.

Ok.

Just to clarify: Remember that you don't need to wait for the release; it's just a formality so people can see an incrementing number and not install autoconf. The patches are already collapsed into master, and I run the entire test suite before committing to that.

Well, you have my e-mail address, if you're willing to share more details privately. ;-)

I think that's fine, but I will release 3.5.7 first.

@ydahhrk ydahhrk added this to the 3.5.7 milestone May 4, 2018

@ydahhrk ydahhrk closed this May 4, 2018

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