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Date:   Mon, 29 Mar 2021 11:38:56 +0200 (CEST)
From:   Richard Biener <rguenther@suse.de>
To:     Jiri Slaby <jslaby@suse.cz>
cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        linux-kernel@vger.kernel.org, "H.J. Lu" <hjl.tools@gmail.com>
Subject: Re: perf does not resolve plt symbols from libstdc++ right (.plt.sec
 problem)
In-Reply-To: <68eb3576-66dd-9155-aae2-b3cde1ebcbdf@suse.cz>
Message-ID: <nycvar.YFH.7.76.2103291136070.17979@zhemvz.fhfr.qr>
References: <d6980662-bf74-1d48-831e-ca1d7209ca2f@suse.cz> <68eb3576-66dd-9155-aae2-b3cde1ebcbdf@suse.cz>
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On Mon, 29 Mar 2021, Jiri Slaby wrote:

> Any ideas on this?
> 
> On 11. 01. 21, 7:31, Jiri Slaby wrote:
> > Hi,
> > 
> > this e-mails is a follow-up of my report at:
> > https://bugzilla.suse.com/show_bug.cgi?id=1180681
> > 
> > There is a problem with *@plt symbols in some libraries, they are unresolved
> > by perf (memcmp@plt in this case):
> >  >???? 0.26%? main2??? /usr/lib64/libstdc++.so.6.0.28??????????? 0xa51a0 
> >  ?????????? l [.] 0x00000000000a51a0
> > 
> > On the other hand, plt symbols in other libraries are fine (memset@plt in
> > this case):
> >  >???? 0.17%? main2??? /usr/lib64/libantlr4-runtime.so.4.8?????? 0x4ed10 
> >  ?????????? l [.] memset@plt
> > 
> > I dumped memcmp's .plt.rela entries in perf:
> > /usr/lib64/libantlr4-runtime.so.4.8: 154th addr=4e9d0 plt_off=4e020 hdr=10
> > entry=10
> > /usr/lib64/libstdc++.so.6.0.28: 772th addr=a1070 plt_off=9e020 hdr=10
> > entry=10
> > 
> > The difference (offset) of stdc++'s memcmp is 0xa51a0 (correct) - 0xa1070
> > (perf's computed) = 0x4130.
> > 
> > The problem is perf assumes nth entry of .plt.rela to correspond to nth
> > function in .plt, but memcmp is in .plt.sec in libstdc++.so:
> > 
> >  >Relocation section '.rela.plt' at offset 0x97900 contains 1018 entries:
> >  >???? Offset???????????? Info???????????? Type?????????????? Symbol's 
> > Value? Symbol's Name + Addend
> >  > ...
> >  > 00000000001dc838? 0000007800000007 R_X86_64_JUMP_SLOT 
> > 0000000000000000 memcmp@GLIBC_2.2.5 + 0
> > 
> > Perf does this with the rela entries:
> > https://github.com/torvalds/linux/blob/f5e6c330254ae691f6d7befe61c786eb5056007e/tools/perf/util/symbol-elf.c#L385 
> > 
> > It takes a symbol index from sym.r_info. Then it resolves its name from
> > .dynsym, appending "@plt" to it. Then this name is added to perf's symbol
> > table along with address which is computed as .rela.plt index multiplied by
> > entry size (shdr_plt.sh_entsize) plus plt header (shdr_plt.sh_entsize on
> > x86_64 too).
> > 
> > And from this comes (almost) the offset above:
> >  >$ objdump -h /usr/lib64/libstdc++.so.6|grep -E ' .plt(\.sec)? '
> >  >? 12 .plt????????? 00003fb0? 000000000009e020? 000000000009e020 
> > 0009e020? 2**4
> >  >? 14 .plt.sec????? 00003fa0? 00000000000a2160? 00000000000a2160 
> > 000a2160? 2**4
> > 
> > 0xa2160-0x9e020 = 0x4140. I assume the 0x10 difference is that perf adds
> > shdr_plt.sh_entsize (0x10) to the offset to skip the first .plt entry
> > (header).
> > 
> > Richard writes:
> > ======
> > .plt.sec is IIRC the "second" (sec) PLT entry - the one that will be used on
> > the second call (and on).? This is used / emitted for ELF object
> > instrumented for Intel CET.? The details escape me for the moment but I hope
> > the x86 ABI documents this (and the constraints) in detail.

I just checked and the x86_64 psABI doesn't say anything about .plt.sec

> > ======
> > 
> > How should perf find out whether to consider .plt or .plt.sec? Or generally,
> > how to properly find an address of *@plt symbols like memcmp@plt above?
> > thanks,
> 
> 
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imend?rffer; HRB 36809 (AG Nuernberg)
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