Issue with $PROGRAM_NAME (sudo + checksum verification)

[michal-josef-spacek]

The issue

The Red Hat customer reported an interesting issue with $PROGRAM_NAME ($0).
There is $PROGRAM_NAME with result like /dev/fd/6 when the script is running under sudo with checksum verification.

We have /usr/local/bin/script.pl

#!/usr/bin/perl

use strict;
use warnings;

use Cwd qw( abs_path );

print "\$0: $0\n";
print "abs_path(\$0): " . abs_path($0) . "\n";

Configuration of sudo:

$ echo "user ALL=NOPASSWD: sha256:$(sha256sum /usr/local/bin/script.pl)" > /etc/sudoers.d/user

Example user:

useradd user

Running as user.

root> su user
user> sudo /usr/local/bin/script.pl

Result like (on Fedora Rawhide):

$0: /dev/fd/6
abs_path($0): /proc/1961/fd/6

I investigated the issue, and there are some consequences:

1. Perl doesn't user prctl API for get of $PROGRAM_NAME
2. C code with prctl works fine in this case.
   C code in ex1.c:

#define _GNU_SOURCE
#include <stdio.h>
#include <string.h>
#include <sys/prctl.h>

int main(void) {
    char name[17] = {0};

    if (prctl(PR_GET_NAME, (unsigned long)name, 0, 0, 0) == -1) {
        perror("prctl(PR_GET_NAME)");
        return 1;
    }

    printf("Name of program: %s\n", name);
    return 0;
}

Compilation:

gcc -o ex1  ex1.c

The result is:

Name of program: ex1

3. When I looked at the code, it is in perl.c:

...
        /* if find_script() returns, it returns a malloc()-ed value */
        scriptname = PL_origfilename = find_script(scriptname, dosearch, NULL, 1);
        s = scriptname + strlen(scriptname);

        if (strBEGINs(scriptname, "/dev/fd/")
            && isDIGIT(scriptname[8])
            && grok_atoUV(scriptname + 8, &uv, &s)
            && uv <= PERL_INT_MAX
        ) {
            fdscript = (int)uv;
            if (*s) {
                /* PSz 18 Feb 04
                 * Tell apart "normal" usage of fdscript, e.g.
                 * with bash on FreeBSD:
                 *   perl <( echo '#!perl -DA'; echo 'print "$0\n"')
                 * from usage in suidperl.
                 * Does any "normal" usage leave garbage after the number???
                 * Is it a mistake to use a similar /dev/fd/ construct for
                 * suidperl?
                 */
                *suidscript = TRUE;
                /* PSz 20 Feb 04
                 * Be supersafe and do some sanity-checks.
                 * Still, can we be sure we got the right thing?
                 */
                if (*s != '/') {
                    Perl_croak(aTHX_ "Wrong syntax (suid) fd script name \"%s\"\n", s);
                }
                if (! *(s+1)) {
                    Perl_croak(aTHX_ "Missing (suid) fd script name\n");
                }
                scriptname = savepv(s + 1);
                Safefree(PL_origfilename);
                PL_origfilename = (char *)scriptname;
            }
        }
    }
... 

The condition:

if (strBEGINs(scriptname, "/dev/fd/")
            && isDIGIT(scriptname[8])
            && grok_atoUV(scriptname + 8, &uv, &s)
            && uv <= PERL_INT_MAX
        ) {

hit the code, but there is no implementation for this situation.

4. I found another situation with similar output

perl <( echo '#!perl -DA'; echo 'print "$0\n"');

The resolution

I tried to implement the resolving of /dev/fd/ to program name via readlink like:

...
                PL_origfilename = (char *)scriptname;
+            } else {
+                char proc_fd_path[64];
+                snprintf(proc_fd_path, sizeof(proc_fd_path), "/proc/self/fd/%d", fdscript);
+                char target_path[PATH_MAX];
+                ssize_t len = readlink(proc_fd_path, target_path, sizeof(target_path) - 1);
+                if (len != -1) {
+                    target_path[len] = '\0';
+                    PL_origfilename = savepv(target_path);
+                }
            }
...

The result in 3) is /usr/local/bin/script.pl
The result in 4) is pipe:number

Questions

What do you think about this?

[michal-josef-spacek]

There is another possible solution in the prctl implementation for get of $PROGRAM_NAME.