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Re: [Qemu-devel] [PATCH] linux-user: Add binfmt wrapper


From: Alexander Graf
Subject: Re: [Qemu-devel] [PATCH] linux-user: Add binfmt wrapper
Date: Mon, 14 Jul 2014 17:46:18 +0200
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:24.0) Gecko/20100101 Thunderbird/24.6.0


On 14.07.14 17:38, Joakim Tjernlund wrote:
Alexander Graf <address@hidden> wrote on 2014/07/14 17:21:33:

From: Alexander Graf <address@hidden>
To: Joakim Tjernlund <address@hidden>,
Cc: address@hidden
Date: 2014/07/14 17:21
Subject: Re: [PATCH] linux-user: Add binfmt wrapper


On 14.07.14 16:38, Joakim Tjernlund wrote:
The popular binfmt-wrapper patch adds an additional
executable which mangle argv suitable for binfmt flag P.
In a chroot you need the both (statically linked) qemu-$arch
and qemu-$arch-binfmt-wrapper. This is sub optimal and a
better approach is to recognize the -binfmt-wrapper extension
within linux-user(qemu-$arch) and mangle argv there.
This just produces on executable which can be either copied to
the chroot or bind mounted with the appropriate -binfmt-wrapper
suffix.

Signed-off-by: Joakim Tjernlund <address@hidden>
Please make sure to CC Riku on patches like this - he is the linux-user
maintainer.
Doesn't he read the devel list? Anyhow CC:ed

He may or may not. Qemu-devel can be pretty high volume :).


---
   linux-user/main.c | 13 +++++++++++++
   1 file changed, 13 insertions(+)

diff --git a/linux-user/main.c b/linux-user/main.c
index 71a33c7..212067a 100644
--- a/linux-user/main.c
+++ b/linux-user/main.c
@@ -3828,6 +3828,19 @@ int main(int argc, char **argv, char **envp)
       int i;
       int ret;
       int execfd;
+    char *binfmt;
+
+    i = strlen( argv[0] ) - strlen ( "-binfmt-wrapper" );
The spaces are odd. Did this patch pass checkpatch.pl? Same comment goes
for almost all function invocations.
ehh, didn't run it through checkpatch.pl. Easy to fix next time.

+    binfmt = argv[0] + i;
+    if (i > 0 && strcmp ( binfmt, "-binfmt-wrapper" ) == 0) {
This magic needs to be documented somewhere. In fact, I find it pretty
hard to use in real world scenarios. Imagine a distribution - should it
package every target binary twice? Should it create hardlinks all over?
How does dists. handle your original binfmt-wrapper? This is not much
different I think. Here you got a choice to create a hardlink or a copy.
Any chroot will only have to bind mount binfmt-wrapper into the chroot or
lxc container.

Yeah, and there are reasons my original approach isn't upstream :).


I think we should try and find better magic :). Looking at the
binfmt_misc loading code, I think we can cheat a bit. If we pass the 'O'
flag (open target binary for handler), binfmt_misc will tell us the
binary fd in AT_EXECFD:

                  NEW_AUX_ENT(AT_EXECFD, bprm->interp_data);

We could then use this as a hint that we were spawned by binfmt_misc
rather than directly and interpret the first argv as target_argv[0].

Then we can also add the P and O flags to scripts/qemu-binfmt-conf.sh
and have a solution that works well for everyone.
What to do with P only then? Seems like most dists uses only P

If a distro uses the P flag it's not using upstream code, so they have to deal with their own breakage :). Fortunately the binfmt install scripts are usually part of a package too, so they can be updated easily.

If a distro cares a lot about backwards compatibility with their old name space, they can still compile the old -binfmt wrapper code and ship it.


Alex




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