Re: [BUG] commit fails with 'bus error' when working directory is on an NFS share

[Dmitriy Panteleyev <dpantel@gmail.com>]

On Wed, Dec 4, 2024 at 8:22 PM Jeff King <peff@peff.net> wrote:
>
> On Wed, Dec 04, 2024 at 07:21:16PM -0700, Dmitriy Panteleyev wrote:
>
> > After trying NO_MMAP=1 with and without SANITIZE, I get:
> > "fatal: mmap failed: Permission denied"
>
> Hmm, that's odd. If you run it under strace, which syscall fails? That
> message should be reporting errno from mmap(), which in NO_MMAP mode
> should be a pread() call. I'm not sure why that would get EACCES if the
> open() call succeeded, but that might explain why the mmap'd version
> gets SIGBUS (I don't know much about NFS, but I imagine that under the
> hood the client is probably issuing reads for individual pages to
> fault in the map).

Strace with NO_MMAP=1, I gives:

openat(AT_FDCWD,
".git/objects/34/5819b235838e219d66420b536a54ce4cf0624c",
O_RDONLY|O_CLOEXEC) = 4
fstat(4, {st_mode=S_IFREG|0444, st_size=154, ...}) = 0
pread64(4, 0x61a0292e15d0, 154, 0)      = -1 ESTALE (Stale file handle)
write(2, "fatal: mmap failed: Permission d"..., 38) = 38

Weirdly, it's throwing ESTALE not EACCESS...

Without NO_MMAP, I get:

openat(AT_FDCWD,
".git/objects/51/da8e85661b60d7378b8ac0d896cfc955405fdf",
O_RDONLY|O_CLOEXEC) = 4
fstat(4, {st_mode=S_IFREG|0444, st_size=154, ...}) = 0
mmap(NULL, 154, PROT_READ, MAP_PRIVATE, 4, 0) = 0x73ceb860e000
close(4)                                = 0
--- SIGBUS {si_signo=SIGBUS, si_code=BUS_ADRERR, si_addr=0x73ceb860e000} ---
+++ killed by SIGBUS (core dumped) +++


Also, it's odd that the same set of commands -- openat(), fstat(), and
pread64() / mmap() -- succeed multiple times before an error is
encountered.


>
> Does your system have AppArmor enabled?

Yes, but I don't see any profiles related to git. And I can't image
AppArmor would be version-dependent.

>
> This issue sounds similar to yours:
>
>   https://unix.stackexchange.com/questions/633389/man-cannot-read-manpage-from-nfs-although-the-file-is-readable
>
> especially the bit where reading the metadata once makes it magically
> work for a brief period (which is the only thing I'd expect the commit
> you found via bisection to have an effect on).
>
> -Peff