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

["Joseph C. Sible" <josephcsible@gmail.com>]

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