Empty /proc/self/auxv (linux-user)

[Thomas Moschny]

The userspace Linux API virtualization used to fake access to /proc/self/auxv, to provide meaningful data for the guest process.

For newer qemu versions, this fails: The openat() is intercepted, but there's no content: /proc/self/auxv has length zero (i.e. reading from it returns 0 bytes).

Good:

$ x86_64-linux-user/qemu-x86_64 /usr/bin/cat /proc/self/auxv | wc -c
256 /proc/self/auxv

Bad:

$ x86_64-linux-user/qemu-x86_64 /usr/bin/cat /proc/self/auxv | wc -c
0 /proc/self/auxv

This worked in 2.7.1, and fails in 2.10.1.

This causes e.g. any procps-ng-based tool to segfault while reading from /proc/self/auxv in an endless loop (probably worth another bug report...)

Doing a "git bisect" shows that this commit: https://github.com/qemu/qemu/commit/7c4ee5bcc introduced the problem.

It might be a simple logic (subtraction in the wrong direction?) or sign-ness error: Adding some logging (to v2.10.1)

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 9b6364a..49285f9 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -7469,6 +7469,9 @@ static int open_self_auxv(void *cpu_env, int fd)
     abi_ulong len = ts->info->auxv_len;
     char *ptr;

+ gemu_log(TARGET_ABI_FMT_lu"\n", len);
+ gemu_log(TARGET_ABI_FMT_ld"\n", len);
+
     /*
      * Auxiliary vector is stored in target process stack.
      * read in whole auxv vector and copy it to file

shows this output:

$ x86_64-linux-user/qemu-x86_64 /usr/bin/cat /proc/self/auxv | wc -c
18446744073709551264
-352
0

And 352 could be the expected length.

[Peter Maydell (pmaydell)]

Oops, yes, commit 7c4ee5bcc82e643 broke this -- it switched the order in which we fill in the AUXV info, but forgot to adjust the calculation of the length, which as you've guessed we now get backwards.

[Peter Maydell (pmaydell)]

I've just sent this patch which fixes this bug:
https://lists.gnu.org/archive/html/qemu-devel/2017-11/msg01199.html
(it turns out it wasn't quite as simple as getting the sign wrong, we were subtracting two things that were totally wrong).

[Thomas Huth (th-huth)]

Fix has been released with QEMU 2.11:
https://git.qemu.org/?p=qemu.git;a=commitdiff;h=f516511ea84d8bb3395d6e