For x86_64, the GNU linker is putting IFUNC information in the relocation
addend, so use it to try to find a symbol for plt entries that refer to
IFUNCs.

Example:

  Before:

    $ cat tstpltlib.c
    void fn1(void) {}
    void fn2(void) {}
    void fn3(void) {}
    void fn4(void) {}
    $ cat tstpltifunc.c
    #include <stdio.h>

    void thing1(void)
    {
            printf("thing1\n");
    }

    void thing2(void)
    {
            printf("thing2\n");
    }

    typedef void (*thing_fn_t)(void);

    thing_fn_t thing_ifunc(void)
    {
            int x;

            if (x & 1)
                    return thing2;
            return thing1;
    }

    void thing(void) __attribute__ ((ifunc ("thing_ifunc")));

    void fn1(void);
    void fn2(void);
    void fn3(void);
    void fn4(void);

    int main()
    {
            fn4();
            fn1();
            thing();
            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 -Wno-uninitialized -o tstpltifunc tstpltifunc.c -L . -ltstpltlib -Wl,-rpath="$(pwd)"
    $ readelf -rW tstpltifunc | grep -A99 plt
    Relocation section '.rela.plt' at offset 0x738 contains 8 entries:
        Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
    0000000000003f98  0000000300000007 R_X86_64_JUMP_SLOT     0000000000000000 puts@GLIBC_2.2.5 + 0
    0000000000003fa8  0000000400000007 R_X86_64_JUMP_SLOT     0000000000000000 __stack_chk_fail@GLIBC_2.4 + 0
    0000000000003fb0  0000000500000007 R_X86_64_JUMP_SLOT     0000000000000000 fn1 + 0
    0000000000003fb8  0000000600000007 R_X86_64_JUMP_SLOT     0000000000000000 fn3 + 0
    0000000000003fc0  0000000800000007 R_X86_64_JUMP_SLOT     0000000000000000 fn4 + 0
    0000000000003fc8  0000000900000007 R_X86_64_JUMP_SLOT     0000000000000000 fn2 + 0
    0000000000003fd0  0000000b00000007 R_X86_64_JUMP_SLOT     0000000000000000 getrandom@GLIBC_2.25 + 0
    0000000000003fa0  0000000000000025 R_X86_64_IRELATIVE                        125d
    $ perf record -e intel_pt//u --filter 'filter main @ ./tstpltifunc' ./tstpltifunc
    thing2
    [ perf record: Woken up 1 times to write data ]
    [ perf record: Captured and wrote 0.016 MB perf.data ]
    $ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
    21860.073683659:   tr strt                               0 [unknown] =>     561e212c42be main+0x0
    21860.073683659:   tr end  call               561e212c42c6 main+0x8 =>     561e212c4110 fn4@plt+0x0
    21860.073683661:   tr strt                               0 [unknown] =>     561e212c42cb main+0xd
    21860.073683661:   tr end  call               561e212c42cb main+0xd =>     561e212c40f0 fn1@plt+0x0
    21860.073683661:   tr strt                               0 [unknown] =>     561e212c42d0 main+0x12
    21860.073683661:   tr end  call               561e212c42d0 main+0x12 =>     561e212c40d0 offset_0x10d0@plt+0x0
    21860.073698451:   tr strt                               0 [unknown] =>     561e212c42d5 main+0x17
    21860.073698451:   tr end  call               561e212c42d5 main+0x17 =>     561e212c4120 fn2@plt+0x0
    21860.073698451:   tr strt                               0 [unknown] =>     561e212c42da main+0x1c
    21860.073698451:   tr end  call               561e212c42da main+0x1c =>     561e212c4100 fn3@plt+0x0
    21860.073698452:   tr strt                               0 [unknown] =>     561e212c42df main+0x21
    21860.073698452:   tr end  return             561e212c42e5 main+0x27 =>     7fb51cc29d90 __libc_start_call_main+0x80

  After:

    $ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
    21860.073683659:   tr strt                               0 [unknown] =>     561e212c42be main+0x0
    21860.073683659:   tr end  call               561e212c42c6 main+0x8 =>     561e212c4110 fn4@plt+0x0
    21860.073683661:   tr strt                               0 [unknown] =>     561e212c42cb main+0xd
    21860.073683661:   tr end  call               561e212c42cb main+0xd =>     561e212c40f0 fn1@plt+0x0
    21860.073683661:   tr strt                               0 [unknown] =>     561e212c42d0 main+0x12
    21860.073683661:   tr end  call               561e212c42d0 main+0x12 =>     561e212c40d0 thing_ifunc@plt+0x0
    21860.073698451:   tr strt                               0 [unknown] =>     561e212c42d5 main+0x17
    21860.073698451:   tr end  call               561e212c42d5 main+0x17 =>     561e212c4120 fn2@plt+0x0
    21860.073698451:   tr strt                               0 [unknown] =>     561e212c42da main+0x1c
    21860.073698451:   tr end  call               561e212c42da main+0x1c =>     561e212c4100 fn3@plt+0x0
    21860.073698452:   tr strt                               0 [unknown] =>     561e212c42df main+0x21
    21860.073698452:   tr end  return             561e212c42e5 main+0x27 =>     7fb51cc29d90 __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/20230131131625.6964-6-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>