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'hard_link = true' deletes special files like '/dev/null' and replaces them with regular files #564

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trofi opened this issue Mar 16, 2020 · 1 comment
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'hard_link = true' deletes special files like '/dev/null' and replaces them with regular files #564

trofi opened this issue Mar 16, 2020 · 1 comment
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bug Does not work as intended/documented
Milestone
3.7.9

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@trofi
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trofi commented Mar 16, 2020

It's an upstream variant of bug https://bugs.gentoo.org/712080 observed and debugged by Dan Goodliffe on ccache-3.7.7

There user runs kernel build as root in a container (or on real machine) and observes /dev/null to be replaced by regular file.

Trigger command:

CCACHE_HARDLINK=1 /usr/lib/ccache/bin/cc -Werror -Wmaybe-uninitialized -S -x c /dev/null -o /dev/null

Kernel's build ssystem probably just probes flag support and redirects unused output to /dev/null.

On my system strace confirms rename attempt. It fails to damage my system only because /dev/null is a mountpoint for me:

# ls -l /dev/null; id; CCACHE_HARDLINK=1 strace -y -f /usr/lib/ccache/bin/cc  -Werror -Wmaybe-uninitialized -S -x c /dev/null -o /dev/null |& egrep 'rename.*null' ; ls -l /dev/null
crw-rw-rw- 1 root root 1, 3 Mar 16 22:17 /dev/null
uid=0(root) gid=0(root) groups=0(root),1(bin),2(daemon),3(sys),4(adm),6(disk),10(wheel),11(floppy),20(dialout),26(tape),27(video)
rename("/root/.ccache/tmp/null.stdout.sf.17002.qbbtTO", "/root/.ccache/tmp/null.stdout.sf.17002.qbbtTO.i") = 0
rename("/dev/null", "/dev/null.rm.sf.17002.XXXXXX") = -1 EBUSY (Device or resource busy)
crw-rw-rw- 1 root root 1, 3 Mar 16 22:17 /dev/null

In upstream bug user observed successful rename():

rename("/dev/null", "/dev/null.rm.akira.261700.XXXXXX") = 0
unlink("/dev/null.rm.akira.261700.XXXXXX") = 0
...

Would it be reasonable for ccache not to try moving special files around even in CCACHE_HARDLINK=1 mode (or maybe handle /dev/null in a special way that does not require writes into target). Or should this behaviour be declared Intended Behaviour?

Thanks!
@trofi trofi added the bug Does not work as intended/documented label Mar 16, 2020
@jrosdahl jrosdahl added this to the 3.7.9 milestone Mar 22, 2020
jrosdahl added a commit to jrosdahl/ccache that referenced this issue Mar 22, 2020
@jrosdahl
Disable hard link mode when the output object file is /dev/null
cb3b6c4 
When hard link mode is enabled, ccache ≥3.6 unlinks the output file
before writing to it as a workaround for a bug in Clang (ccache#331). This
unfortunately means that /dev/null will be removed when building as root
(don’t do that, BTW) with hard link mode enabled and /dev/null as the
the output file.

Fix this by disabling hard link mode if the output object file is
/dev/null and by not copying cached output to /dev/null at all, thus not
triggering unlink-before-write for other files (e.g. dependency file) as
well. (There is no need to handle other non-regular output files since
/dev/null is the only allowed non-regular output file.)

Fixes ccache#564.
jrosdahl added a commit to jrosdahl/ccache that referenced this issue Mar 22, 2020
@jrosdahl
Disable hard link mode when the output object file is /dev/null
0db43db 
When hard link mode is enabled, ccache ≥3.6 unlinks the output file
before writing to it as a workaround for a bug in Clang (ccache#331). This
unfortunately means that /dev/null will be removed when building as root
(don’t do that, BTW) with hard link mode enabled and /dev/null as the
the output file. A similar problem exists if the dependency file is
/dev/null, regardless of hard link mode.

Fix this by not unlinking the output file if it’s /dev/null and by not
copying files to /dev/null at all. (There is no need to handle other
non-regular output files since /dev/null is the only allowed non-regular
output file.)

Fixes ccache#564.
jrosdahl added a commit to jrosdahl/ccache that referenced this issue Mar 22, 2020
@jrosdahl
Disable hard link mode when the output object file is /dev/null
9a79468 
When hard link mode is enabled, ccache ≥3.6 unlinks the output file
before writing to it as a workaround for a bug in Clang (ccache#331). This
unfortunately means that /dev/null will be removed when building as root
(don’t do that, BTW) with hard link mode enabled and /dev/null as the
the output file. A similar problem exists if the dependency file is
/dev/null, regardless of hard link mode.

Fix this by not unlinking the output file if it’s /dev/null and by not
copying files to /dev/null at all. (There is no need to handle other
non-regular output files since /dev/null is the only allowed non-regular
output file.)

Fixes ccache#564.
@jrosdahl
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Fixed by 9a79468 on 3.7-maint and 1905d63 on master.

@gwnichol gwnichol mentioned this issue Apr 5, 2021
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