qemu user mode does not support binfmt_misc config with flags include "P"

[Luyou Peng]

Hi Sir:
During our test in chroot environment with qemu-user-static, we got some test cases failed because of program output warning with unexpected full path name.
For example in test module "Devscripts"
the test item for broken tarball expected the warning info:
<tar: This does not look like a tar archive
tar: ******* >
but the output was:
</bin/tar: This does not look like a tar archive
/bin/tar: ******>
the cause is the config file of binfmt_misc was set not to send argv0, for example:
type command "tar" after chroot:
==========================
lpeng@lpeng-VirtualBox:~/projects_lpeng/qemu/mips_2/sid$ sudo chroot .
[sudo] password for lpeng:
root@lpeng-VirtualBox:/# tar
/bin/tar: You must specify one of the '-Acdtrux', '--delete' or '--test-label' options
Try '/bin/tar --help' or '/bin/tar --usage' for more information.
root@lpeng-VirtualBox:/#
===========================

by adding output log in main()@qemu/Linux-user/main.c
we found the original input command was changed, and qemu do not know that, we got the input args:
argv_0----/usr/bin/qemu-mips64el-static---
argv_1----/bin/tar---
argv_2----NULL---

Next step we modified the flags=P in the corresponding config under folder /proc/sys/fs/binfmt_misc, then binfmt_misc sent argv[0] to qemu.
But chroot could not start bash because in current qemu dose not consider about this unexpected one more"argv[0]"

After modified qemu code temporary to handle the new argv list we got the input args, and from argv[2] is the original input command
argv_0----/usr/bin/qemu-mips64el-static---
argv_1----/bin/tar---
argv_2----tar---

We need the original input from command line, so is it possible that let binfmt_misc to pass one more additional args or env to qemu as a token of the binfmt_misc flag, then qemu can judge how to parse the input args by it?
looking forward your suggestions.

Thanks
luyou

[Luyou Peng (lpeng)]

I modified binfmt_misc.c to let it send one more args if the corresponding binfmt_misc flags include "P", also I modified main.c in qemu as attached patch_qemu_main.patch to check if the input args include such string. Then qemu user could support both scenarios.
Is this modification feasible?

[Peter Maydell (pmaydell)]

Doesn't doing the check like that break execution of a command like "echo 'MISC_FMT_PRESERVE_ARGV0'" in a chroot environment that isn't using -P ?

[Luyou Peng (lpeng)]

hi Peter:
Thanks for your suggestion.
but anyway we have to modify qemu code when binfmt_misc passes argv[0] in, right?

[Peter Maydell (pmaydell)]

The question is: can we have one set of QEMU code that copes correctly with both 'binfmt_misc with P flag' and 'binfmt_misc without P flag' ?

Your patch makes -P work but breaks some cases without it. That means it's not backwards compatible with all the existing QEMU installations and use cases in the world, which means it's awkward to move to. It would be better if there were a mechanism which would allow us to make them both work.

[Luyou Peng (lpeng)]

hi Peter:
You are right, any additional modification should not affect the previous.
We expect that if this issue could be resolved without code change it's the best.
it may need to modify both binfmt_misc side to pass more information for flag P and qemu side to handle it.

[YunQiang Su (wzssyqa)]

@Peter Luyou and me are working on try to pass the info about whether P flag is enabled or not by enviroment var or auxval. While we have not found the right method to do it from binfmt_misc.

In fact, currently qemu trys to process the O flag, and it cannot work at all.
When you install qemu-user-static package from Debian/Ubuntu, the O flag is enabled,
while
   execfd = qemu_getauxval(AT_EXECFD);
always return 0.

[YunQiang Su (wzssyqa)]

This patch is for linux kernel.

It will set the 3rd bit of AT_FLAGS, if P is set for binfmt_misc.

The major concern is that AT_FLAGS is never used, then, should we use it here?

[YunQiang Su (wzssyqa)]

This patch is for qemu itself.

It test AT_FLAGS and determine whether it is start by binfmt_misc and whether P flag is used.

[Thomas Huth (th-huth)]

The QEMU project is currently considering to move its bug tracking to
another system. For this we need to know which bugs are still valid
and which could be closed already. Thus we are setting older bugs to
"Incomplete" now.

If you still think this bug report here is valid, then please switch
the state back to "New" within the next 60 days, otherwise this report
will be marked as "Expired". Or please mark it as "Fix Released" if
the problem has been solved with a newer version of QEMU already.

Thank you and sorry for the inconvenience.

[Launchpad Janitor (janitor)]

[Expired for QEMU because there has been no activity for 60 days.]