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

[Rich Felker <dalias@libc.org>]

On Sun, Sep 02, 2018 at 01:42:26PM -0400, Joseph C. Sible wrote:
> On Sun, Sep 2, 2018 at 1:11 PM Rich Felker <dalias@libc.org> wrote:
> >
> > 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
> 
> Good catch. Updated patch below.
> -- >8 --
> Subject: [PATCH v2] fexecve: implement in terms of execveat when it exists
> 
> This lets fexecve work even when /proc isn't mounted.
> ---
>  src/process/fexecve.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/src/process/fexecve.c b/src/process/fexecve.c
> index 6507b42..5cac05e 100644
> --- a/src/process/fexecve.c
> +++ b/src/process/fexecve.c
> @@ -1,10 +1,15 @@
> +#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[])
>  {
> +	int r = __syscall(SYS_execveat, fd, "", argv, envp, AT_EMPTY_PATH);
> +	if(r != -ENOSYS) return __syscall_ret(r);
>  	char buf[15 + 3*sizeof(int)];
>  	__procfdname(buf, fd);
>  	execve(buf, argv, envp);
> -- 
> 2.7.4

Thanks, merging. In the future please send patches as attachments
(preferably type text/plain if it's easy to pick) unless you're sure
they'll apply correctly with git-am from the body.

Rich