Re: [PATCH] syscmd: Allow commands with leading - or +

[Eric Blake]

On Fri, Nov 19, 2021 at 02:58:27PM -0600, Eric Blake wrote:
> As POSIX recently pointed out[1], anything with semantics like
> system() or popen() should be passing "--" between "-c" and the user's
> string, in case the user intends to execute a utility beginning with
> '-' or '+'.  POSIX recommends that users should not name files
> beginning with '-', but does not have a similar discouragement against
> files beginning with '+'.  In particular, if your /bin/sh is bash, and
> you want to execute a script named "+O", this patch is essential.  If
> you need to be portable to older m4, you can always prepend a space in
> your arguments to syscmd().
> 
> [1] https://www.austingroupbugs.net/view.php?id=1440
> 
> * src/builtin.c (m4_syscmd, m4_esyscmd): Pass "--" to sh prior to
> user's string.
> ---
>  src/builtin.c | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)

I forgot to squash this in before sending my email:

* NEWS: Add a line.
---
 NEWS | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/NEWS b/NEWS
index f4d59cd0..910a6f68 100644
--- a/NEWS
+++ b/NEWS
@@ -2,6 +2,8 @@ GNU M4 NEWS - User visible changes.

 * Noteworthy changes in release ?.? (????-??-??) [?]

+** The `syscmd' and `esyscmd' builtins no longer mishandle a command line
+   starting with `-' or `+'.

 * Noteworthy changes in release 1.4.19 (2021-05-28) [stable]

-- 
2.33.1



> 
> diff --git a/src/builtin.c b/src/builtin.c
> index 5e36ab35..576aebcb 100644
> --- a/src/builtin.c
> +++ b/src/builtin.c
> @@ -947,7 +947,8 @@ m4_syscmd (struct obstack *obs MAYBE_UNUSED, int argc, 
> token_data **argv)
>    const char *cmd = ARG (1);
>    int status;
>    int sig_status;
> -  const char *prog_args[4] = { "sh", "-c" };
> +  int slot = 3;
> +  const char *prog_args[5] = { "sh", "-c", "--" };
>    if (bad_argc (argv[0], argc, 2, 2) || !*cmd)
>      {
>        /* The empty command is successful.  */
> @@ -961,9 +962,10 @@ m4_syscmd (struct obstack *obs MAYBE_UNUSED, int argc, 
> token_data **argv)
>      {
>        prog_args[0] = "cmd";
>        prog_args[1] = "/c";
> +      slot = 2;
>      }
>  #endif
> -  prog_args[2] = cmd;
> +  prog_args[slot] = cmd;
>    errno = 0;
>    status = execute (ARG (0), SYSCMD_SHELL, prog_args, NULL, false,
>                      false, false, false, true, false, &sig_status);
> @@ -984,12 +986,13 @@ static void
>  m4_esyscmd (struct obstack *obs, int argc, token_data **argv)
>  {
>    const char *cmd = ARG (1);
> -  const char *prog_args[4] = { "sh", "-c" };
> +  const char *prog_args[5] = { "sh", "-c", "--" };
>    pid_t child;
>    int fd;
>    FILE *pin;
>    int status;
>    int sig_status;
> +  int slot = 3;
> 
>    if (bad_argc (argv[0], argc, 2, 2) || !*cmd)
>      {
> @@ -1004,9 +1007,10 @@ m4_esyscmd (struct obstack *obs, int argc, token_data 
> **argv)
>      {
>        prog_args[0] = "cmd";
>        prog_args[1] = "/c";
> +      slot = 2;
>      }
>  #endif
> -  prog_args[2] = cmd;
> +  prog_args[slot] = cmd;
>    errno = 0;
>    child = create_pipe_in (ARG (0), SYSCMD_SHELL, prog_args, NULL,
>                            NULL, false, true, false, &fd);
> -- 
> 2.33.1
> 
> 
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> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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