Fix PR breakpoints/16297: catch syscall with syscall 0

Code rationale
==============
by: Gabriel Krisman Bertazi

This is a fix for bug 16297. The problem occurs when the user attempts
to catch any syscall 0 (such as syscall read on Linux/x86_64). GDB was
not able to catch the syscall and was missing the breakpoint.

Now, breakpoint_hit_catch_syscall returns immediately when it finds the
correct syscall number, avoiding a following check for the end of the
search vector, that returns a no hit if the syscall number was zero.

Testcase rationale
==================
by: Sergio Durigan Junior

This testcase is a little difficult to write.  By doing a quick
inspection at the Linux source, one can see that, in many targets, the
syscall number 0 is restart_syscall, which is forbidden to be called
from userspace.  Therefore, on many targets, there's just no way to test
this safely.

My decision was to take the simpler route and just adds the "read"
syscall on the default test.  Its number on x86_64 is zero, which is
"good enough" since many people here do their tests on x86_64 anyway and
it is a popular architecture.

However, there was another little gotcha.  When using "read" passing 0
as the third parameter (i.e., asking it to read 0 bytes), current libc
implementations could choose not to effectively call the syscall.
Therefore, the best solution was to create a temporary pipe, write 1
byte into it, and then read this byte from it.

gdb/ChangeLog
2013-12-19  Gabriel Krisman Bertazi  <gabriel@krisman.be>

	PR breakpoints/16297
	* breakpoint.c (breakpoint_hit_catch_syscall): Return immediately
	when expected syscall is hit.

gdb/testsuite/ChangeLog
2013-12-19  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR breakpoints/16297
	* gdb.base/catch-syscall.c (read_syscall, pipe_syscall)
	(write_syscall): New variables.
	(main): Create a pipe, write 1 byte in it, and read 1 byte from
	it.
	* gdb.base/catch-syscall.exp (all_syscalls): Include "pipe,
	"write" and "read" syscalls.
	(fill_all_syscalls_numbers): Improve the way to obtain syscalls
	numbers.

1 files changed, 3 insertions, 4 deletions

diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index 589aa19..6a11ddf 100644
--- a/gdb/breakpoint.c
+++ b/gdb/breakpoint.c
@@ -8325,10 +8325,9 @@ breakpoint_hit_catch_syscall (const struct bp_location *bl,
            VEC_iterate (int, c->syscalls_to_be_caught, i, iter);
            i++)
 	if (syscall_number == iter)
-	  break;
-      /* Not the same.  */
-      if (!iter)
-	return 0;
+	  return 1;
+
+      return 0;
     }
 
   return 1;