From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Christoph Hellwig <hch@lst.de>,
Randy Dunlap <rdunlap@infradead.org>,
Andrew Morton <akpm@linux-foundation.org>,
broonie@kernel.org, linux-fsdevel@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
linux-next@vger.kernel.org, mhocko@suse.cz,
mm-commits@vger.kernel.org, sfr@canb.auug.org.au,
Linus Torvalds <torvalds@linux-foundation.org>,
viro@zeniv.linux.org.uk, x86@kernel.org,
Steven Rostedt <rostedt@goodmis.org>
Subject: Re: mmotm 2020-05-13-20-30 uploaded (objtool warnings)
Date: Fri, 29 May 2020 11:50:11 -0500 [thread overview]
Message-ID: <20200529165011.o7vvhn4wcj6zjxux@treble> (raw)
In-Reply-To: <20200529161253.GD706460@hirez.programming.kicks-ass.net>
On Fri, May 29, 2020 at 06:12:53PM +0200, Peter Zijlstra wrote:
> On Fri, May 29, 2020 at 11:05:14AM -0500, Josh Poimboeuf wrote:
>
> > It looks to me like GCC is doing the right thing. That likely()
> > translates to:
> >
> > # define likely(x) (__branch_check__(x, 1, __builtin_constant_p(x)))
> >
> > which becomes:
> >
> > #define __branch_check__(x, expect, is_constant) ({ \
> > long ______r; \
> > static struct ftrace_likely_data \
> > __aligned(4) \
> > __section(_ftrace_annotated_branch) \
> > ______f = { \
> > .data.func = __func__, \
> > .data.file = __FILE__, \
> > .data.line = __LINE__, \
> > }; \
> > ______r = __builtin_expect(!!(x), expect); \
> > ftrace_likely_update(&______f, ______r, \
> > expect, is_constant); \
> > ______r; \
> > })
> >
> > Here 'x' is the call to user_access_begin(). It evaluates 'x' -- and
> > thus calls user_access_begin() -- before the call to
> > ftrace_likely_update().
> >
> > So it's working as designed, right? The likely() just needs to be
> > changed to likely_notrace().
>
> But if !x (ie we fail user_access_begin()), we should not pass STAC() on
> the way to out_err. OTOH if x, we should not be jumping to out_err.
>
> I'm most confused... must not stare at asm for a while.
Yeah, I saw that call to ftrace_likely_update() and got distracted. I
forgot it's on the uaccess safe list.
From staring at the asm I think the generated code is correct, it's just
that the nested likelys with ftrace profiling cause GCC to converge the
error/success paths. But objtool doesn't do register value tracking so
it's not smart enough to know that it's safe.
The nested likelys seem like overkill anyway -- user_access_begin() is
__always_inline and it already has unlikely(), which should be
propagated.
So just remove the outer likelys?
diff --git a/arch/x86/lib/csum-wrappers_64.c b/arch/x86/lib/csum-wrappers_64.c
index a12b8629206d..ee63d7576fd2 100644
--- a/arch/x86/lib/csum-wrappers_64.c
+++ b/arch/x86/lib/csum-wrappers_64.c
@@ -27,7 +27,7 @@ csum_and_copy_from_user(const void __user *src, void *dst,
might_sleep();
*errp = 0;
- if (!likely(user_access_begin(src, len)))
+ if (!user_access_begin(src, len))
goto out_err;
/*
@@ -89,7 +89,7 @@ csum_and_copy_to_user(const void *src, void __user *dst,
might_sleep();
- if (unlikely(!user_access_begin(dst, len))) {
+ if (!user_access_begin(dst, len)) {
*errp = -EFAULT;
return 0;
}
next prev parent reply other threads:[~2020-05-29 16:50 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-14 0:50 incoming Andrew Morton
2020-05-14 0:50 ` [patch 1/7] mm, memcg: fix inconsistent oom event behavior Andrew Morton
2020-05-14 0:50 ` [patch 2/7] epoll: call final ep_events_available() check under the lock Andrew Morton
2020-05-14 0:50 ` [patch 3/7] mm/gup: fix fixup_user_fault() on multiple retries Andrew Morton
2020-05-14 0:50 ` [patch 4/7] userfaultfd: fix remap event with MREMAP_DONTUNMAP Andrew Morton
2020-05-14 0:50 ` [patch 5/7] ipc/util.c: sysvipc_find_ipc() incorrectly updates position index Andrew Morton
2020-05-14 0:50 ` [patch 6/7] kasan: consistently disable debugging features Andrew Morton
2020-05-14 0:50 ` [patch 7/7] kasan: add missing functions declarations to kasan.h Andrew Morton
2020-05-14 3:31 ` mmotm 2020-05-13-20-30 uploaded Andrew Morton
2020-05-14 15:30 ` mmotm 2020-05-13-20-30 uploaded (gpu/drm/bridge/sil-sii8620) Randy Dunlap
2020-05-14 15:32 ` mmotm 2020-05-13-20-30 uploaded (objtool warnings) Randy Dunlap
2020-05-14 15:33 ` Randy Dunlap
2020-05-28 15:54 ` Josh Poimboeuf
2020-05-28 17:04 ` Josh Poimboeuf
2020-05-28 17:20 ` Peter Zijlstra
2020-05-29 13:57 ` Christoph Hellwig
2020-05-29 14:35 ` Peter Zijlstra
2020-05-29 14:53 ` Peter Zijlstra
2020-05-29 15:33 ` Peter Zijlstra
2020-05-29 16:05 ` Josh Poimboeuf
2020-05-29 16:12 ` Peter Zijlstra
2020-05-29 16:50 ` Josh Poimboeuf [this message]
2020-05-29 16:54 ` Peter Zijlstra
2020-05-29 17:25 ` [PATCH] x86/uaccess: Remove redundant likely/unlikely annotations Josh Poimboeuf
2020-05-29 18:29 ` Randy Dunlap
2020-05-29 19:31 ` mmotm 2020-05-13-20-30 uploaded (objtool warnings) Linus Torvalds
2020-05-29 20:08 ` Al Viro
2020-05-29 20:14 ` Al Viro
2020-05-15 23:30 ` mmotm 2020-05-15-16-29 uploaded Andrew Morton
2020-05-16 5:53 ` Stephen Rothwell
2020-05-18 23:15 ` Stephen Rothwell
2020-05-20 4:48 ` mmotm 2020-05-19-21-47 uploaded Andrew Morton
2020-05-22 3:43 ` mmotm 2020-05-21-20-42 uploaded Andrew Morton
2020-05-22 16:26 ` mmotm 2020-05-21-20-42 uploaded (atomisp) Randy Dunlap
2020-05-23 3:36 ` mmotm 2020-05-22-20-35 uploaded Andrew Morton
2020-05-23 15:08 ` mmotm 2020-05-22-20-35 uploaded (phy/intel/phy-intel-combo.c) Randy Dunlap
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20200529165011.o7vvhn4wcj6zjxux@treble \
--to=jpoimboe@redhat.com \
--cc=akpm@linux-foundation.org \
--cc=broonie@kernel.org \
--cc=hch@lst.de \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=linux-next@vger.kernel.org \
--cc=mhocko@suse.cz \
--cc=mm-commits@vger.kernel.org \
--cc=peterz@infradead.org \
--cc=rdunlap@infradead.org \
--cc=rostedt@goodmis.org \
--cc=sfr@canb.auug.org.au \
--cc=torvalds@linux-foundation.org \
--cc=viro@zeniv.linux.org.uk \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).