Re: [PATCH 1/2] run-command: Add checks after execvp fails with EACCES

Thanks for the review. There's a lot of things you mention that I
either didn't see (staring blind, you know) or that I didn't know of.

On Tue, Dec 6, 2011 at 11:35 PM, Junio C Hamano [off-list ref] wrote:

Frans Klaver [off-list ref] writes:

quoted

+#ifndef WIN32
+static int is_in_group(gid_t gid)
...
+static int have_read_execute_permissions(const char *path)
+{
+     struct stat s;
+     trace_printf("checking '%s'\n", path);
+
+     if (stat(path, &s) < 0) {
+ ...
+     /* check world permissions */
+     if ((s.st_mode&(S_IXOTH|S_IROTH)) == (S_IXOTH|S_IROTH))
+             return 1;

Hmm, do you need to do this with stat(2)?

Wouldn't access(2) with R_OK|X_OK give you exactly what you want without
this much trouble?

Probably. I'll use access instead in a reroll.

I also think that your permission check is incorrectly implemented.

   $ cd /var/tmp && date >j && chmod 044 j && ls -l j
   ----r--r-- 1 junio junio 29 Dec  6 14:32 j
   $ cat j
   cat: j: Permission denied
   $ su pogo
   Password:
   $ cat j
   Tue Dec  6 14:32:23 PST 2011

That's a world-readable but unreadable-only-to-me file.

Hmm, this is a case that didn't fit my expectations. Thanks for catching.

quoted

+static void diagnose_execvp_eacces(const char *cmd, const char **argv)
+{
+     /* man 2 execve states that EACCES is returned for:

       /*
        * Just a style, but we tend to write multi-line comment like
        * this, without anything else on opening and closing lines of
        * the comment block.
        */

quoted

+      * - The file system is mounted noexec
+      */
+     struct strbuf sb = STRBUF_INIT;
+     char *path = getenv("PATH");
+     char *next;
+
+     if (strchr(cmd, '/')) {
+             if (!have_read_execute_permissions(cmd))
+                     error("no read/execute permissions on '%s'\n", cmd);
+             return;
+     }

Ok, execvp() failed and "cmd" has at least one slash, so we know we did
not look for it in $PATH.  We check only one and return (did you need
getenv() in that case?).

Obviously not. Missed that.

quoted

+     for (;;) {
+             next = strchrnul(path, ':');
+             if (path < next)
+                     strbuf_add(&sb, path, next - path);
+             else
+                     strbuf_addch(&sb, '.');

Nice touch that you did not forget an empty component on $PATH.

Yes, that's a relic from me starting work based on one of your
proposed patches[1]. So that one goes to you.

[1] http://article.gmane.org/gmane.comp.version-control.git/171838

quoted

+             if (!have_read_execute_permissions(sb.buf))
+                     error("no read/execute permissions on '%s'\n", sb.buf);

Don't you want to continue here upon error, after resetting sb? You just
saw the directory is unreadble, so you know next file_exists() will fail
before you try it.

Yes. I thought about that. I didn't do that because of the fact that I
had to do more than just resetting sb. The path variable has to be
updated as well. I had the choice of adding a level of indentation {},
duplicating the code, or just do a check I know before will fail.
There's probably something to say for each one of them. I'll probably
refactor that a bit more.

quoted

+             if (sb.len && sb.buf[sb.len - 1] != '/')
+                     strbuf_addch(&sb, '/');
+             strbuf_addstr(&sb, cmd);
+
+             if (file_exists(sb.buf)) {
+                     if (!have_read_execute_permissions(sb.buf))
+                             error("no read/execute permissions on '%s'\n",
+                                             sb.buf);
+                     else
+                             warn("file '%s' exists and permissions "
+                             "seem OK.\nIf this is a script, see if you "
+                             "have sufficient privileges to run the "
+                             "interpreter", sb.buf);

Does "warn()" do the right thing for multi-line strings like this?

I don't know/remember. It seemed like a natural thing to do, but I'll find out.