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


From: Philippe Mathieu-Daudé
Subject: Re: linux-user: keep the name-ending parenthesis in /proc/self/stat
Date: Tue, 31 Mar 2020 00:05:34 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

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<address@hidden>

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;





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