Re: linux-user: keep the name-ending parenthesis in /proc/self/stat

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 3/30/20 9:07 PM, Brice Goglin wrote:
> When the program name is very long, qemu-user may truncate it in
> /proc/self/stat. However the truncation must keep the ending ") "
> to conform to the proc manpage which says:
>      (2) comm  %s
>             The  filename of the executable, in parentheses.  This
>             is visible whether or not the  executable  is  swapped
>             out.
> 
> To reproduce:
> $ ln -s /bin/cat <filenamewithmorethan128chars>
> $ qemu-x86_64 ./<filenamewithmorethan128chars> /proc/self/stat
> 
> Before the patch, you get:
> 1134631 (<filenametruncated>0 0 0 0 0 0 0 0 ...
> After the patch:
> 1134631 (<filenametruncat>) 0 0 0 0 0 0 0 0 ...
> 
> This fixes an issue with hwloc failing to parse /proc/self/stat
> when Ludovic Courtes was testing it in guix over qemu-aarch64.
> 
> Signed-off-by: Brice Goglin<Brice.Goglin@inria.fr>
> 
> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> index 5af55fca78..a1126dcf5b 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -7305,7 +7305,10 @@ static int open_self_stat(void *cpu_env, int fd)
>           snprintf(buf, sizeof(buf), "%"PRId64 " ", val);
>         } else if (i == 1) {
>           /* app name */
> -        snprintf(buf, sizeof(buf), "(%s) ", ts->bprm->argv[0]);
> +        len = snprintf(buf, sizeof(buf), "(%s) ", ts->bprm->argv[0]);
> +        if (len >= sizeof(buf))
> +          /* bring back the ending ") " that was truncated */
> +          strcpy(buf+sizeof(buf)-3, ") ");

Maybe we can avoid the sprintf() call:

-- >8 --
diff --git a/linux-user/syscall.c b/linux-user/syscall.c
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -7305,7 +7305,11 @@ static int open_self_stat(void *cpu_env, int fd)
          snprintf(buf, sizeof(buf), "%"PRId64 " ", val);
        } else if (i == 1) {
          /* app name */
-        snprintf(buf, sizeof(buf), "(%s) ", ts->bprm->argv[0]);
+        char *ptr = buf;
+
+        *ptr++ = '(';
+        ptr = stpncpy(ptr, ts->bprm->argv[0], sizeof(buf) - 3);
+        strcpy(ptr, ") ");
        } else if (i == 27) {
          /* stack bottom */
          val = start_stack;
---

>         } else if (i == 27) {
>           /* stack bottom */
>           val = start_stack;
>