* [musl] [PATCH] fix fexecve fallback code when fd is negative
@ 2021-07-08 0:15 Érico Nogueira
2021-07-08 1:39 ` Rich Felker
0 siblings, 1 reply; 2+ messages in thread
From: Érico Nogueira @ 2021-07-08 0:15 UTC (permalink / raw)
To: musl; +Cc: Érico Nogueira
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
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [musl] [PATCH] fix fexecve fallback code when fd is negative
2021-07-08 0:15 [musl] [PATCH] fix fexecve fallback code when fd is negative Érico Nogueira
@ 2021-07-08 1:39 ` Rich Felker
0 siblings, 0 replies; 2+ messages in thread
From: Rich Felker @ 2021-07-08 1:39 UTC (permalink / raw)
To: musl
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
^ permalink raw reply [flat|nested] 2+ messages in thread
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2021-07-08 0:15 [musl] [PATCH] fix fexecve fallback code when fd is negative Érico Nogueira
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