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Message-ID: <20210708013943.GS13220@brightrain.aerifal.cx>
Date: Wed, 7 Jul 2021 21:39:43 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] fix fexecve fallback code when fd is negative
On Wed, Jul 07, 2021 at 09:15:19PM -0300, Érico Nogueira wrote:
> even though passing negative fd values to fexecve shouldn't happen on
> the part of any application, if it does happen and the current kernel
> doesn't implement SYS_execveat, musl will call execve on
> "/proc/self/fd/(unsigned)fd", which might be an existing, unrelated,
> file.
>
> this was introduced in commit c8c0844f7fbcb955848ca84432e5ffcf71f1cef1,
> when fexecve started using __procfdname, which takes an unsigned integer
> instead of signed one.
>
> this also fixes the POSIX-compliance of the function, since POSIX
> specifies:
>
> The fexecve() function shall fail if:
>
> EBADF The fd argument is not a valid file descriptor open for
> executing.
>
> this corner case was found while investigating usage of __procfdname.
> all other cases are correct:
>
> - fchmod,fchdir,fchown -> will fail the direct syscall first
> - fchmodat -> is in the *at family so fd can be -1, only uses procfdname with another fd
> - fstatat -> checks for fd>=0 in the branch that uses procfdname
> - ttyname_r -> fd goes through isatty()
> ---
> src/process/fexecve.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/src/process/fexecve.c b/src/process/fexecve.c
> index 554c1981..ab81b2d3 100644
> --- a/src/process/fexecve.c
> +++ b/src/process/fexecve.c
> @@ -8,6 +8,8 @@ int fexecve(int fd, char *const argv[], char *const envp[])
> {
> int r = __syscall(SYS_execveat, fd, "", argv, envp, AT_EMPTY_PATH);
> if (r != -ENOSYS) return __syscall_ret(r);
> + else if (fd < 0) return __syscall_ret(-EBADF);
> +
> char buf[15 + 3*sizeof(int)];
> __procfdname(buf, fd);
> execve(buf, argv, envp);
> --
> 2.32.0
To document what we discussed on IRC, if x has type int and x<0,
(unsigned)x > INT_MAX, so it cannot masquerade as a value in the range
0..INT_MAX (which a fd must be). This means the code is already
working, and the line following the above context:
if (errno == ENOENT) errno = EBADF;
handles conversion to the expected error code.
Rich
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