linux-kernel - Re: [fs/exec] 80bd5afdd8: xfstests.generic.633.fail

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Date:   Mon, 31 Jan 2022 17:14:15 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     kernel test robot <oliver.sang@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Ariadne Conill <ariadne@...eferenced.org>,
        0day robot <lkp@...el.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Rich Felker <dalias@...c.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        stable@...r.kernel.org
Subject: Re: [fs/exec]  80bd5afdd8: xfstests.generic.633.fail

On Mon, Jan 31, 2022 at 03:51:21PM +0000, Matthew Wilcox wrote:
> On Mon, Jan 31, 2022 at 04:37:07PM +0100, Christian Brauner wrote:
> > On Mon, Jan 31, 2022 at 03:19:22PM +0000, Matthew Wilcox wrote:
> > > On Mon, Jan 31, 2022 at 04:08:19PM +0100, Christian Brauner wrote:
> > > > On Mon, Jan 31, 2022 at 10:43:52PM +0800, kernel test robot wrote:
> > > > I can fix this rather simply in our upstream fstests with:
> > > > 
> > > > static char *argv[] = {
> > > > 	"",
> > > > };
> > > > 
> > > > I guess.
> > > > 
> > > > But doesn't
> > > > 
> > > > static char *argv[] = {
> > > > 	NULL,
> > > > };
> > > > 
> > > > seem something that should work especially with execveat()?
> > > 
> > > The problem is that the exec'ed program sees an argc of 0, which is the
> > > problem we're trying to work around in the kernel (instead of leaving
> > > it to ld.so to fix for suid programs).
> > 
> > Ok, just seems a bit more intuitive for path-based exec than for
> > fd-based execveat().
> > 
> > What's argv[0] supposed to contain in these cases?
> > 
> > 1. execveat(fd, NULL, ..., AT_EMPTY_PATH)
> > 2. execveat(fd, "my-file", ..., )
> > 
> > "" in both 1. and 2.?
> > "" in 1. and "my-file" in 2.?
> 
> You didn't specify argv for either of those, so I have no idea.
> Programs shouldn't be assuming anything about argv[0]; it's purely
> advisory.  Unfortunately, some of them do.  And some of them are suid.

Yes, programs shouldn't assume anything about argv[0]. But a lot of
programs are used to setting argv[0] to the name of the executed binary.
The exec* manpages examples do this. Just looking at a random selftest, e.g.

bpf/prog_tests/test_lsm.c

where we find:

	char *CMD_ARGS[] = {"true", NULL};
	execvp(CMD_ARGS[0], CMD_ARGS);

I'm just wondering how common this is for execveat() because it is not
as clear what the actual name of the binary is in these two examples

	1.
	fd = open("/bin/true", );
	char *CMD_ARGS[] = {"", NULL};
	execveat(fd, NULL, ..., AT_EMPTY_PATH)
	
	2.
	fd = open("/bin", );
	char *CMD_ARGS[] = {"true", NULL};
	execveat(fd, CMD_ARGS[0], CMD_ARGS 0)

in other words, the changes that you see CMD_ARGS[0] == NULL for
execveat() seem higher than for path-based exec.

To counter that we should probably at least update the execveat()
manpage with a recommendation what CMD_ARGS[0] should be set to if it
isn't allowed to be set to NULL anymore. This is why was asking what
argv[0] is supposed to be if the binary doesn't take any arguments.