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References: <20230127170222.9895-1-adrian.hunter@intel.com>
 <20230127170222.9895-3-adrian.hunter@intel.com> <CAM9d7chny89+rtP6orSJOKrAqb4GbzCG53KV4O1J9mhKvCcWnQ@mail.gmail.com>
 <5aeab2b5-7349-d220-652b-75360ef6d723@intel.com>
In-Reply-To: <5aeab2b5-7349-d220-652b-75360ef6d723@intel.com>
From:   Namhyung Kim <namhyung@kernel.org>
Date:   Mon, 30 Jan 2023 14:22:59 -0800
Message-ID: <CAM9d7ch2T1B_A_meyZi5A+kpwmzCY-eAKuZv5OVYGonBrhXqgQ@mail.gmail.com>
Subject: Re: [PATCH 2/9] perf symbols: Add support for x86 .plt.sec
To:     Adrian Hunter <adrian.hunter@intel.com>
Cc:     Arnaldo Carvalho de Melo <acme@kernel.org>,
        Jiri Olsa <jolsa@redhat.com>, Ian Rogers <irogers@google.com>,
        linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org
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On Mon, Jan 30, 2023 at 10:35 AM Adrian Hunter <adrian.hunter@intel.com> wrote:
>
> On 30/01/23 19:34, Namhyung Kim wrote:
> > Hi Adrian,
> >
> > On Fri, Jan 27, 2023 at 9:02 AM Adrian Hunter <adrian.hunter@intel.com> wrote:
> >>
> >> The section .plt.sec was originally added for MPX and was first called
> >> .plt.bnd. While MPX has been deprecated, .plt.sec is now also used for IBT.
> >> On x86_64, IBT seems to be enabled by default, but can be switched off
> >> using gcc option -fcf-protection=none. On 32-bit, option -z ibt will
> >> enable IBT.
> >>
> >> With .plt.sec, calls are made into .plt.sec instead of .plt, so it
> >> makes more sense to put the symbols there instead of .plt. A notable
> >> difference is that .plt.sec does not have a header entry.
> >>
> >> For x86, when synthesizing symbols for plt, use offset and entry size of
> >> .plt.sec instead of .plt when there is a .plt.sec section.
> >>
> >> Example on Ubuntu 22.04 gcc 11.3:
> >>
> >>   Before:
> >>
> >>     $ cat tstpltlib.c
> >>     void fn1(void) {}
> >>     void fn2(void) {}
> >>     void fn3(void) {}
> >>     void fn4(void) {}
> >>     $ cat tstplt.c
> >>     void fn1(void);
> >>     void fn2(void);
> >>     void fn3(void);
> >>     void fn4(void);
> >>
> >>     int main()
> >>     {
> >>             fn4();
> >>             fn1();
> >>             fn2();
> >>             fn3();
> >>             return 0;
> >>     }
> >>     $ gcc --version
> >>     gcc (Ubuntu 11.3.0-1ubuntu1~22.04) 11.3.0
> >>     Copyright (C) 2021 Free Software Foundation, Inc.
> >>     This is free software; see the source for copying conditions.  There is NO
> >>     warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
> >>     $ gcc -Wall -Wextra -shared -o libtstpltlib.so tstpltlib.c
> >>     $ gcc -Wall -Wextra -o tstplt tstplt.c -L . -ltstpltlib -Wl,-rpath=$(pwd)
> >>     $ readelf -SW tstplt | grep 'plt\|Name'
> >>       [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
> >>       [11] .rela.plt         RELA            0000000000000698 000698 000060 18  AI  6  24  8
> >>       [13] .plt              PROGBITS        0000000000001020 001020 000050 10  AX  0   0 16
> >>       [14] .plt.got          PROGBITS        0000000000001070 001070 000010 10  AX  0   0 16
> >>       [15] .plt.sec          PROGBITS        0000000000001080 001080 000040 10  AX  0   0 16
> >
> > On my machine, it's not enabled by default.  And it doesn't create .plt.sec
> > even if I pass -fcf-protection=full option.
> >
> > $ gcc --version
> > gcc (Debian 12.2.0-10) 12.2.0
> > Copyright (C) 2022 Free Software Foundation, Inc.
> > This is free software; see the source for copying conditions.  There is NO
> > warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
> >
> > $ gcc -Wall -Wextra -shared -fcf-protection=full -o libtstplt.so tstpltlib.c
> > $ gcc -Wall -Wextra -fcf-protection=full -o tstplt tstplt.c -L.
> > -ltstpltlib -Wl,-rpath,$(pwd)
> > $ readelf -SW tstplt | grep 'plt\|Name'
> >    [Nr] Name              Type            Address          Off    Size
> >   ES Flg Lk Inf Al
> >    [11] .rela.plt         RELA            0000000000000688 000688
> > 000060 18  AI  6  24  8
> >    [13] .plt              PROGBITS        0000000000001020 001020
> > 000050 10  AX  0   0 16
> >    [14] .plt.got          PROGBITS        0000000000001070 001070
> > 000008 08  AX  0   0  8
> >    [24] .got.plt          PROGBITS        0000000000003fe8 002fe8
> > 000038 08  WA  0   0  8
>
> That is interesting.  What does it say with -v i.e.
>
>         gcc -v -Wall -Wextra -o tstplt tstplt.c -L . -ltstpltlib -Wl,-rpath=$(pwd)

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>
> And what is the distribution?

It's a Debian (Testing) with some customization.

Thanks,
Namhyung