Access to /proc/self/exe in linux-user mode

[Yury Gribov]

This is based on a recent bug in GCC Bugzilla: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60681

It looks like libbacktrace (GCC runtime library used for obtaining stack traces) uses /proc/self/exe for error reporting. Currently this is mapped to qemu-arm which effectively disables libbacktrace on linux-user.

It seems that QEMU already supports /proc/self/{maps,stat,auxv} so addition of /proc/self/exe may be trivial.

[Maxim Ostapenko (m-ostapenko)]

This tiny patch seems to work.

[Yury Gribov (tetra2005)]

I think the problem is not in libbacktrace per se but rather libsanitizer initializing libbacktrace with contents of /proc/self/exe. Patch is still relevant though.

[Riku Voipio (riku-voipio)]

Looks good, I'll get this to linux-user que once QEMU 2.0 is released.

[Peter Maydell (pmaydell)]

That patch will copy the whole of the target executable into a temporary file without changing any of it -- the fake_open mechanism is really intended for cases where we need to return modified results. Wouldn't it be easier to just have something in do_open() that said:
    if (is_proc_myself(pathname, "exe")) {
        return get_errno(open(exec_path), flags, mode);
    }

That will then give the right behaviour for read-only executables and other error-related corner cases.

(See also the logic in the readlink/readlinkat handling which already specialcases /proc/self/exe using exec_path.)

[Peter Maydell (pmaydell)]

(I got the bracket placement wrong there so as you can tell the code is untested :-))

[Maxim Ostapenko (m-ostapenko)]

Yes, it works. Here is updated patch.

[Peter Maydell (pmaydell)]

Some nits:
 The "(CPUArchState *)" cast isn't necessary
 We should use exec_path, not ts->bprm->argv[0] (the guest argv[0] isn't necessarily the executable path)
 We don't want to call path() here -- exec_path is a host path, and only guest filename paths need to go through path().

Looking a little more closely at the logic in main.c I wonder if we actually want:

   if (is_proc_myself(pathname, "exe")) {
        execfd = qemu_getauxval(AT_EXECFD);
        if (execfd) {
             return execfd;
        }
        return get_errno(open(exec_path, flags, mode));
    }

Also if you'd like us to apply your patches we'll need at least a "Signed-off-by: " line from you.

[Maxim Ostapenko (m-ostapenko)]

Ok, fixed.

[Peter Maydell (pmaydell)]

Thanks. That version
Reviewed-by: Peter Maydell <email address hidden>

[Maxim Ostapenko (m-ostapenko)]

Hi,

Is this patch deployed in new version of QEMU?

Thanks,
Maxim

[Peter Maydell (pmaydell)]

This bug was fixed by commit aa07f5ecf9828 in 2014 and has been released in QEMU.