[LTP] [PATCH] lib: tst_fd: Avoid tst_brk(TCONF, ...) on older distros

[Petr Vorel]

> Hi!
> > Actually, the solution I posted [1] works on both old (affected) kernel and new
> > one:

> > -	fd->fd = syscall(__NR_pidfd_open, getpid(), 0);
> > +	fd->fd = tst_syscall(__NR_pidfd_open, getpid(), 0);
> >  	if (fd->fd < 0)
> >  		tst_brk(TBROK | TERRNO, "pidfd_open()");

> This cannot work on kenrel that does not implement pidfd_open. That's
> what the code in tst_syscall() does:

> #define tst_syscall(NR, ...) ({ \
>         intptr_t tst_ret; \
>         if (NR == __LTP__NR_INVALID_SYSCALL) { \
>                 errno = ENOSYS; \
>                 tst_ret = -1; \
>         } else { \
>                 tst_ret = syscall(NR, ##__VA_ARGS__); \
>         } \
>         if (tst_ret == -1 && errno == ENOSYS) { \
>                 TST_SYSCALL_BRK__(NR, #NR); \
>         } \
>         tst_ret; \
> })

> This means that either if the syscall number is undefined or if the
> actuall syscall fails with ENOSYS we call tst_brk(TCONF, ...) or
> tst_brkm(TCONF, ...) on old API.

> > I guess we should merge your solution (otherwise we would need to change other
> > cases to be consistent), but I'm a bit confused. Is it the reason why we use
> > syscall() + tst_res(TCONF) instead of tst_syscall() + tst_brk(TBROK) that for
> > some cases it's expected to fail, thus TBROK is not accepted? Again, I was blind
> > when doing review.

> The problem is that if kernel does not implement a particular syscall
> the tst_syscall() calls tst_brk() which ends the tst_fd iteration in the
> middle. The tst_fd iterator just loop over different types of file
> descriptors, if you call tst_brk() anywhere the test ends before we
> managed to finish the loop. We do not want that to happen because of
> either syscall not implemented in older kernels or syscalls disabled by
> CONFIG options.

> That's why we have to call syscall() and do tst_res(TCONF, ...) when the
> syscall had failed. The tst_fd_next() will just continue with next fd
> type if we failed to produce a valid fd.

Understand, you want to keep tst_res(), that makes sense.
And it does not make sense whether the failure is due ENOSYS or due other
failure (most likely due expected error).

Kind regards,
Petr