linux-kernel - Re: [PATCH 2/9] perf symbols: Add support for x86 .plt.sec

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Date:   Mon, 30 Jan 2023 20:35:03 +0200
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, Ian Rogers <irogers@...gle.com>,
        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org
Subject: Re: [PATCH 2/9] perf symbols: Add support for x86 .plt.sec

On 30/01/23 19:34, Namhyung Kim wrote:
> Hi Adrian,
> 
> On Fri, Jan 27, 2023 at 9:02 AM Adrian Hunter <adrian.hunter@...el.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)

And what is the distribution?