Re: [BUG] Core dump w/ bus error on bad mmap'd packfile

Amit Bakshi [off-list ref] writes:

 I was getting core dumps (bus error 7) with git 1.7.1 when doing a
git checkout. I tried git fsck, and same thing. I got the same with
precompiled rpms of 1.7.12, and with a locally compiled version of git
1.8.5.

[...]

$ git fsck
Checking pack-0ccb5ee4f60003a7ea47d6904633cce5973efa60.pack
in: 0x7f9070507000 remain: 17345210 offset: 17345230
Bus error (core dumped)

git verify-pack correctly shows the error, without a bus error.

$ git verify-pack -v pack-0ccb5ee4f60003a7ea47d6904633cce5973efa60.pack
fatal: read error on input: Input/output error
pack-0ccb5ee4f60003a7ea47d6904633cce5973efa60.pack: bad

mmap()ing a file and attempting to read from parts that the kernel can
then not page in (in your case because it couldn't read the file) are
expected to give a SIGBUS, so there is no bug here.  The different
behavior comes from some commands using mmap() and some using read() to
access the packs.

Perhaps we could try to map the faulting address back to the file it
came from, though.  The siginfo_t for a SIGBUS contains information
about the faulting address, so for the case of packs we could walk the
'struct packed_git' list and see which open window the address belongs
to.  I need to think about the implications of doing that in a signal
handler, though.

-- 
Thomas Rast
tr@thomasrast.ch