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

[Namhyung Kim]

Hi Adrian,

On Fri, Jan 27, 2023 at 9:02 AM Adrian Hunter <adrian.hunter@xxxxxxxxx> 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

Thanks,
Namhyung