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.
