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Date: Thu, 25 May 2023 10:53:09 +0300
From: Alexey Izbyshev <izbyshev@...ras.ru>
To: musl@...ts.openwall.com
Subject: getopt_long() can corrupt argv when an argument for a short option is
missing
POSIX requires getopt() to set optind to argc + 1 in case of a missing
argument[1], and musl follows it. This bites getopt_long() (which reuses
getopt()) in two ways:
* getopt_long() moves argv[optind - 1] (NULL) when permuting argv to
make all options precede other arguments, essentially corrupting argv.
* even when permuting is not required, getopt_long() is both
incompatible with glibc (which doesn't increment optind past NULL) and
inconsistent with itself (for a long option with a missing argument,
musl doesn't increment optind past NULL too).
Example of the wrong NULL shifting:
#include <getopt.h>
#include <stdio.h>
int main(int argc, char *argv[]) {
for (int i = 0; i < 2; i++) {
int r = getopt_long(argc, argv, "o:", NULL, NULL);
printf("r: %d\n", r);
printf("optind: %d\n", optind);
for (int i = 0; i <= argc; i++)
printf("%d: '%s'\n", i, argv[i]);
}
}
With glibc:
$ ./a.out arg -o
./a.out: option requires an argument -- 'o'
r: 63
optind: 3
0: './a.out'
1: 'arg'
2: '-o'
3: '(null)'
r: -1
optind: 2
0: './a.out'
1: '-o'
2: 'arg'
3: '(null)'
(Note that glibc permutes argv *before* parsing then next option, and
even before comparing optind and argc, so argv is still permuted on the
second invocation.)
With musl:
$ ./a.out arg -o
./a.out: option requires an argument: o
r: 63
optind: 3
0: './a.out'
1: '-o'
2: '(null)'
3: 'arg'
r: -1
optind: 3
0: './a.out'
1: '-o'
2: '(null)'
3: 'arg'
Maybe we could just skip permuting and adjust optind if we detected a
missing argument?
resumed = optind;
ret = __getopt_long_core(argc, argv, optstring, longopts, idx,
longonly);
+ if (optind > argc)
+ return optind--, ret;
if (resumed > skipped) {
On a subsequent invocation we won't permute, unlike glibc, but maybe
this is a good thing, given that such permutation makes it look like
there is no missing argument, essentially changing the command
semantics.
Alexey
[1]
https://pubs.opengroup.org/onlinepubs/9699919799/functions/getopt.html
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