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Re: [PATCH 0/2] wait/ptrace: always assume __WALL if the child is traced
- From: Pedro Alves <palves at redhat dot com>
- To: Oleg Nesterov <oleg at redhat dot com>, Andrew Morton <akpm at linux-foundation dot org>, Dmitry Vyukov <dvyukov at google dot com>
- Cc: Alexander Potapenko <glider at google dot com>, Denys Vlasenko <dvlasenk at redhat dot com>, Eric Dumazet <edumazet at google dot com>, Jan Kratochvil <jan dot kratochvil at redhat dot com>, Julien Tinnes <jln at google dot com>, Kees Cook <keescook at google dot com>, Kostya Serebryany <kcc at google dot com>, Linus Torvalds <torvalds at linux-foundation dot org>, "Michael Kerrisk (man-pages)" <mtk dot manpages at gmail dot com>, Robert Swiecki <swiecki at google dot com>, Roland McGrath <roland at hack dot frob dot com>, syzkaller at googlegroups dot com, linux-kernel at vger dot kernel dot org, "gdb at sourceware dot org" <gdb at sourceware dot org>
- Date: Thu, 22 Oct 2015 15:40:41 +0100
- Subject: Re: [PATCH 0/2] wait/ptrace: always assume __WALL if the child is traced
- Authentication-results: sourceware.org; auth=none
- References: <20151020171740 dot GA29290 at redhat dot com>
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