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From: Nick Desaulniers <ndesaulniers@google.com>
To: Fangrui Song <maskray@google.com>
Cc: Kees Cook <keescook@chromium.org>, "KE . LI" <like1@oppo.com>,
	Nathan Chancellor <nathan@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	Miroslav Benes <mbenes@suse.cz>,
	"Gustavo A. R. Silva" <gustavoars@kernel.org>,
	Stephen Boyd <swboyd@chromium.org>,
	Sami Tolvanen <samitolvanen@google.com>,
	Joe Perches <joe@perches.com>,
	linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com
Subject: Re: [PATCH] kallsyms: strip LTO suffixes from static functions
Date: Mon, 28 Jun 2021 11:20:41 -0700	[thread overview]
Message-ID: <CAKwvOdnyzRk75+P1Uc=hs8e2aL3oMSDvjy-JeYwXwJwd_G7JRw@mail.gmail.com> (raw)
In-Reply-To: <CAKwvOdki=HZh4TYwqwDSo4BWtbGHp6pM_2akA+D3K8JO+dMGoQ@mail.gmail.com>

On Mon, Jun 28, 2021 at 10:54 AM Nick Desaulniers
<ndesaulniers@google.com> wrote:
>
> On Tue, Jun 22, 2021 at 1:18 PM Fangrui Song <maskray@google.com> wrote:
> >
> > On 2021-06-22, 'Nick Desaulniers' via Clang Built Linux wrote:
> > >+/*
> > >+ * LLVM mangles static functions for full LTO so that two static functions with
> > >+ * the same identifier do not collide when all code is combined into one
> > >+ * module. The scheme used converts references to foo into
> > >+ * foo.llvm.974640843467629774, for example. This can break hooking of static
> > >+ * functions with kprobes.
> > >+ */
> >
> > The comment should say ThinLTO instead.
> >
> > The .llvm.123 suffix is for global scope promotion for local linkage
> > symbols. The scheme is ThinLTO specific. This ensures that a local
>
> Oh, boy. Indeed.  I had identified the mangling coming from
> getGlobalNameForLocal(), but looking at the call chain now I see:
>
> FunctionImportGlobalProcessing::processGlobalForThinLTO()
> -> FunctionImportGlobalProcessing::getPromotedName()
>   -> ModuleSummaryIndex::getGlobalNameForLocal()
>
> I'm not sure then how I figured it was specific to full LTO.
>
> Android recently switched from thin LTO to full LTO, which is what I
> assumed was the cause of the bug report. Rereading our internal bug
> report, it was tested against a prior version that did the symbol
> truncation for thinLTO. I then assumed this was full LTO specific for
> whatever reason, and modified the patch to only apply to full LTO.  I
> see via the above call chain that this patch is not correct.  Let me
> send my original patch as a v2. b/189560201 if you're interested.

I can even see the .llvm.<number> symbol names via `llvm-nm` on
vmlinux for thinLTO builds. No such symbols exist for full LTO.

-- 
Thanks,
~Nick Desaulniers

  reply	other threads:[~2021-06-28 18:21 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-22 18:38 Nick Desaulniers
2021-06-22 20:18 ` Fangrui Song
2021-06-28 17:54   ` Nick Desaulniers
2021-06-28 18:20     ` Nick Desaulniers [this message]
2021-06-28 19:05   ` [PATCH v2] " Nick Desaulniers
2021-06-28 19:45     ` Nathan Chancellor
2021-06-28 20:31       ` [PATCH v3] " Nick Desaulniers
2021-06-28 21:19         ` Nathan Chancellor
2021-06-28 22:01           ` Nick Desaulniers
2021-06-28 22:16             ` Nathan Chancellor
2021-07-07 18:18               ` [PATCH v4] " Nick Desaulniers
2021-07-07 18:34                 ` Nathan Chancellor
2021-07-07 18:59                   ` Fāng-ruì Sòng
2021-08-06 16:20                 ` Sami Tolvanen
2021-10-01 19:58                   ` [PATCH v5] " Nick Desaulniers
2021-10-01 20:05                     ` Sami Tolvanen
2021-10-04 10:46                       ` Padmanabha Srinivasaiah

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