Re: [PATCH 32/32] execve.2: Fix absolute/relative pathname

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hello Alex, Shawn, and Nora,

@Nora, thanks for the bug report.

On 7/28/21 10:20 PM, Alejandro Colomar wrote:
> From: Shawn Landden <shawn@git.icu>
> 
> The absolute pathname is available in the Linux-specific
> auxiliary-vector feature.

@Shawn
But this isn't true, as far as I can see. See the following:

[[
$ cat auxv.c 
#include <link.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

extern char **environ;

int
main(int argc, char *argv[])
{
    char **p;

    /* Walk through the array of pointers in environ */

    for (p = environ; *p != NULL; p++)
        continue;

    /* The auxiliary vector sits just above environ */

    p++;        /* Step past NULL pointer at end of 'environ' */

    printf("auxvec                        %12p\n", (void *) p);

    ElfW(auxv_t) *auxv = (ElfW(auxv_t) *) p;

    while (auxv->a_type != 0) {

        switch (auxv->a_type) {
        case AT_PLATFORM:
                        /* String identifying hardware platform */
            printf("AT_PLATFORM:      0x%llx (string: %s)\n",
                    (long long) auxv->a_un.a_val, (char *) auxv->a_un.a_val);
            break;
        case AT_SECURE: /* Was secure-execution mode triggered for dynamic
                           linker? See ld.so(8). */
            printf("AT_SECURE:        %lld\n", (long long) auxv->a_un.a_val);
            break;
        case AT_EXECFN: /* Pathname used to execute program */
            printf("AT_EXECFN:        0x%llx (string: %s)\n",
                    (long long) auxv->a_un.a_val, (char *) auxv->a_un.a_val);
            break;
        default:
            /* We ignore other types of entries in auxiliary vector */
            break;
        }

        auxv++;
    }

    exit(EXIT_SUCCESS);
}
$ ./auxv
auxvec                        0x7ffd5c6eb708
AT_SECURE:        0
AT_EXECFN:        0x7ffd5c6ecff1 (string: ./auxv)
AT_PLATFORM:      0x7ffd5c6eb859 (string: x86_64)
]]

> 
> Reported-by: Nora Platiel <nplatiel@gmx.us>
> Signed-off-by: Shawn Landden <shawn@git.icu>
> Reviewed-by: Nora Platiel <nplatiel@gmx.us>
> Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
> ---
>  man2/execve.2 | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/man2/execve.2 b/man2/execve.2
> index c18ca5412..7fe12ab42 100644
> --- a/man2/execve.2
> +++ b/man2/execve.2
> @@ -346,7 +346,9 @@ will be invoked with the following arguments:
>  .PP
>  where
>  .I pathname
> -is the absolute pathname of the file specified as the first argument of
> +is the absolute pathname of the file specified (may be relative to
> +.BR getcwd (3)
> +of the caller) as the first argument of
>  .BR execve (),

I find the "(may be relative...)" piece confusing.

It's simpler just to s/absolute pathname/pathname/, and that's the
change I made.

>  and
>  .I arg...
> @@ -356,6 +358,11 @@ argument of
>  .BR execve (),
>  starting at
>  .IR argv[1] .
> +The absolute pathname of the script is also available
> +in the same auxiliary vector the environment and argument variables are in, as
> +.BR AT_EXECFN .
> +See
> +.BR getauxval (3).
>  Note that there is no way to get the
>  .IR argv[0]
>  that was passed to the

I applied the patch below.

Cheers,

Michael

diff --git a/man2/execve.2 b/man2/execve.2
index 435a6b363..4eaf1967e 100644
--- a/man2/execve.2
+++ b/man2/execve.2
@@ -346,7 +346,7 @@ will be invoked with the following arguments:
 .PP
 where
 .I pathname
-is the absolute pathname of the file specified as the first argument of
+is the pathname of the file specified as the first argument of
 .BR execve (),
 and
 .I arg...


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/