linux-kernel - Re: [PATCH 0/2] wait/ptrace: always assume _

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Date:	Thu, 22 Oct 2015 15:40:41 +0100
From:	Pedro Alves <palves@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitry Vyukov <dvyukov@...gle.com>
CC:	Alexander Potapenko <glider@...gle.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Jan Kratochvil <jan.kratochvil@...hat.com>,
	Julien Tinnes <jln@...gle.com>,
	Kees Cook <keescook@...gle.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Robert Swiecki <swiecki@...gle.com>,
	Roland McGrath <roland@...k.frob.com>,
	syzkaller@...glegroups.com, linux-kernel@...r.kernel.org,
	"gdb@...rceware.org" <gdb@...rceware.org>
Subject: Re: [PATCH 0/2] wait/ptrace: always assume __WALL if the child is
 traced

On 10/20/2015 06:17 PM, Oleg Nesterov wrote:

> Jan, Pedro, could you please confirm this won't break gdb? I tried
> to look into gdb-7.1, and at first glance gdb uses __WCLONE only
> because __WALL doesn't work on older kernels, iow it seems to me
> that gdb actually wants __WALL so this change should be fine.

Right, gdb actually wants __WALL, but it doesn't use it to keep
compatibility with kernels that predate it.

gdb nowadays has an __WALL emulation waitpid wrapper
(alternates __WCLONE+WNOHANG with 0+WNOHANG, blocks
on sigsuspend/SIGCHLD):

 https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=gdb/nat/linux-waitpid.c;h=cbcdd95afa9c664993542b0b3851e79fbae4e1df;hb=HEAD#l77

Though it's not used everywhere.  Some older code in the
ptrace backend open codes the "try __WCLONE, then try !__WCLONE."
dance.

Seems like __WALL was added in Linux 2.4; gdb could probably
assume it's available nowadays...

In any case, to make sure existing gdb binaries would still work
with your kernel change, I ran GDB's testsuite with this:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/gdb/nat/linux-waitpid.c b/gdb/nat/linux-waitpid.c
index cbcdd95..864ba2e 100644
--- a/gdb/nat/linux-waitpid.c
+++ b/gdb/nat/linux-waitpid.c
@@ -149,3 +149,17 @@ my_waitpid (int pid, int *status, int flags)
   errno = out_errno;
   return ret;
 }
+
+#include <dlfcn.h>
+
+pid_t
+waitpid (pid_t pid, int *status, int options)
+{
+  static pid_t (*waitpid2) (pid_t pid, int *status, int options) = NULL;
+
+  if (waitpid2 == NULL)
+    waitpid2 = dlsym (RTLD_NEXT, "waitpid");
+
+  options |= __WALL;
+  return waitpid2 (pid, status, options);
+}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

and got no regressions.  So seems like all would be well from
GDB's perspective.

Thanks,
Pedro Alves

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