Re: [PATCH] fexecve: implement in terms of execveat when it exists

[Rich Felker <dalias@libc.org>]

On Sun, Sep 02, 2018 at 12:04:55AM -0400, Joseph Sible wrote:
> This lets fexecve work even when /proc isn't mounted.
> ---
>  src/process/fexecve.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/src/process/fexecve.c b/src/process/fexecve.c
> index 6507b42..905487e 100644
> --- a/src/process/fexecve.c
> +++ b/src/process/fexecve.c
> @@ -1,13 +1,20 @@
> +#define _GNU_SOURCE
>  #include <unistd.h>
>  #include <errno.h>
> +#include <fcntl.h>
> +#include "syscall.h"
> 
>  void __procfdname(char *, unsigned);
> 
>  int fexecve(int fd, char *const argv[], char *const envp[])
>  {
> +#ifdef SYS_execveat
> +       return syscall(SYS_execveat, fd, "", argv, envp, AT_EMPTY_PATH);
> +#else
>         char buf[15 + 3*sizeof(int)];
>         __procfdname(buf, fd);
>         execve(buf, argv, envp);
>         if (errno == ENOENT) errno = EBADF;
>         return -1;
> +#endif
>  }
> --
> 2.7.4

This breaks programs running on any kernel older than 3.19.

Instead it needs to be something like

	int r = __syscall(SYS_execveat, fd, "", argv, envp, AT_EMPTY_PATH);
	if (r!=-ENOSYS) return __syscall_ret(r);
	...

with no #ifdef. #ifdef SYS_anything is only valid in musl when the
existence of the syscall is arch-specific. The defines come from musl
itself, so trying to use it for something version-specific does not
make sense; it would be unconditionally true or false.

Rich