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 may be enabled by default, but can be switched off
using gcc option -fcf-protection=none, or switched on by -z ibt or -z
ibtplt. On 32-bit, option -z ibt or -z ibtplt 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 -z ibt -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
    $ perf record -e intel_pt//u --filter 'filter main @ ./tstplt' ./tstplt
    [ perf record: Woken up 1 times to write data ]
    [ perf record: Captured and wrote 0.015 MB perf.data ]
    $ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
    38970.522546686:   tr strt                               0 [unknown] =>     55fc222a81a9 main+0x0
    38970.522546686:   tr end  call               55fc222a81b1 main+0x8 =>     55fc222a80a0 [unknown]
    38970.522546687:   tr strt                               0 [unknown] =>     55fc222a81b6 main+0xd
    38970.522546687:   tr end  call               55fc222a81b6 main+0xd =>     55fc222a8080 [unknown]
    38970.522546688:   tr strt                               0 [unknown] =>     55fc222a81bb main+0x12
    38970.522546688:   tr end  call               55fc222a81bb main+0x12 =>     55fc222a80b0 [unknown]
    38970.522546688:   tr strt                               0 [unknown] =>     55fc222a81c0 main+0x17
    38970.522546688:   tr end  call               55fc222a81c0 main+0x17 =>     55fc222a8090 [unknown]
    38970.522546689:   tr strt                               0 [unknown] =>     55fc222a81c5 main+0x1c
    38970.522546894:   tr end  return             55fc222a81cb main+0x22 =>     7f3a4dc29d90 __libc_start_call_main+0x80

  After:

    $ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
    38970.522546686:   tr strt                               0 [unknown] =>     55fc222a81a9 main+0x0
    38970.522546686:   tr end  call               55fc222a81b1 main+0x8 =>     55fc222a80a0 fn4@plt+0x0
    38970.522546687:   tr strt                               0 [unknown] =>     55fc222a81b6 main+0xd
    38970.522546687:   tr end  call               55fc222a81b6 main+0xd =>     55fc222a8080 fn1@plt+0x0
    38970.522546688:   tr strt                               0 [unknown] =>     55fc222a81bb main+0x12
    38970.522546688:   tr end  call               55fc222a81bb main+0x12 =>     55fc222a80b0 fn2@plt+0x0
    38970.522546688:   tr strt                               0 [unknown] =>     55fc222a81c0 main+0x17
    38970.522546688:   tr end  call               55fc222a81c0 main+0x17 =>     55fc222a8090 fn3@plt+0x0
    38970.522546689:   tr strt                               0 [unknown] =>     55fc222a81c5 main+0x1c
    38970.522546894:   tr end  return             55fc222a81cb main+0x22 =>     7f3a4dc29d90 __libc_start_call_main+0x80
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230131131625.6964-3-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>