semantic: 0.700
other: 0.582
mistranslation: 0.574
instruction: 0.522
device: 0.396
graphic: 0.335
network: 0.333
boot: 0.296
vnc: 0.276
socket: 0.269
assembly: 0.224
KVM: 0.173

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

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.

This tiny patch seems to work.

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.

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

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.)


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


Yes, it works. Here is updated patch.

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.


Ok, fixed.

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


Hi,

Is this patch deployed in new version of QEMU?

Thanks,
Maxim

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