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If lfstack gets any pointer on amd64 platforms where the top bit is set then an assert is thrown.
In most cases this won't occur since the kernel will consume the top bit of VA, leaving the userspace with 47bits of VA.
This is a logic bug around address canonicalization.
I don't have a basic application or environment for reproducing this. Easy to see the logic bug.
What did you expect to see?
While it is correct that you must canonicalize the pointer by sign extending it on 48bit systems (I'm not even going to bring up LA57). The check inside of lfstack is incorrect since it assumes the sign extension will never occur. Thus assuming it will never receive a true 48-bit pointer.
This assumption is broken in environments where you get 48-bit pointers with the high bit set.
This is purely a simple logic bug, it shouldn't assert in this case.
What did you see instead?
An assert on 48-bit address with high bit set.
The text was updated successfully, but these errors were encountered:
Only a partial fix for FEX-Emu#1330, still needs preemption disabled to work.
On x86-64 hosts the Linux kernel resides in the top bit of VA which
isn't mapped in to userspace.
This means that userspace will never receive pointers living with that
top bit set unless you're running a 57bit VA host.
This results in userspace pointers never needing to do the sign
extending pointer canonicalization. But additionally some applications
actually don't understand the pointer canonicalization.
This results in bugs like: golang/go#49405
Now if you're running on a 57bit VA host, this will end up behaving like
FEX but it seems like no one in golang land has really messed with 57bit
In AArch64, when configured with a 48bit VA, the userspace gets the full
48bit VA space and on EL mode switch has the full address range change
to the kernel's 48bit VA.
This means that we will /very/ likely allocate pointers in the high
48bit space since Linux currently allocates top-down.
So behave more like x86-64, hide the top 128TB of memory space from the
guest before boot.
Testing: Took the M1Max 15ms to 21ms allocate the top 128TB.
Currently the only real concern is if anyone tries running on Intel's latest server platforms with LA57 enabled.
Which doesn't pass the userspace a 48-bit (or higher) pointer unless explicitly asked.
Otherwise it would likely just be toy environments attempting a 48-bit VA space like AArch64 platforms. Switching memory mappings on CPL change.