perf symbols: Allow for static executables with .plt

A statically linked executable can have a .plt due to IFUNCs, in which
case .symtab is used not .dynsym. Check the section header link to see
if that is the case, and then use symtab instead.

Example:

Before:

$ cat tstifunc.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")));

int main()
{
thing();
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 -static -Wall -Wextra -Wno-uninitialized -o tstifuncstatic tstifunc.c
$ readelf -SW tstifuncstatic | grep 'Name\|plt\|dyn'
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[ 4] .rela.plt RELA 00000000004002e8 0002e8 000258 18 AI 29 20 8
[ 6] .plt PROGBITS 0000000000401020 001020 000190 00 AX 0 0 16
[20] .got.plt PROGBITS 00000000004c5000 0c4000 0000e0 08 WA 0 0 8
$ perf record -e intel_pt//u --filter 'filter main @ ./tstifuncstatic' ./tstifuncstatic
thing1
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.008 MB perf.data ]
$ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
15786.690189535: tr strt 0 [unknown] => 4017cd main+0x0
15786.690189535: tr end call 4017d5 main+0x8 => 401170 [unknown]
15786.690197660: tr strt 0 [unknown] => 4017da main+0xd
15786.690197660: tr end return 4017e0 main+0x13 => 401c1a __libc_start_call_main+0x6a

After:

$ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
15786.690189535: tr strt 0 [unknown] => 4017cd main+0x0
15786.690189535: tr end call 4017d5 main+0x8 => 401170 thing_ifunc@plt+0x0
15786.690197660: tr strt 0 [unknown] => 4017da main+0xd
15786.690197660: tr end return 4017e0 main+0x13 => 401c1a __libc_start_call_main+0x6a

Reviewed-by: Namhyung Kim <[e-mail]>
Signed-off-by: Adrian Hunter <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/symbol-elf.c

···
 	GElf_Shdr shdr_rel_plt, shdr_dynsym;
 	Elf_Data *syms, *symstrs;
 	Elf_Scn *scn_plt_rel, *scn_symstrs, *scn_dynsym;
-	size_t dynsym_idx;
 	GElf_Ehdr ehdr;
 	char sympltname[1024];
 	Elf *elf;
···
 		lazy_plt = true;
 	}
 
-	scn_dynsym = ss->dynsym;
-	shdr_dynsym = ss->dynshdr;
-	dynsym_idx = ss->dynsym_idx;
-
-	if (scn_dynsym == NULL)
-		return 0;
-
 	scn_plt_rel = elf_section_by_name(elf, &ehdr, &shdr_rel_plt,
 					  ".rela.plt", NULL);
 	if (scn_plt_rel == NULL) {
···
 	    shdr_rel_plt.sh_type != SHT_REL)
 		return 0;
 
-	if (shdr_rel_plt.sh_link != dynsym_idx)
+	if (!shdr_rel_plt.sh_link)
+		return 0;
+
+	if (shdr_rel_plt.sh_link == ss->dynsym_idx) {
+		scn_dynsym = ss->dynsym;
+		shdr_dynsym = ss->dynshdr;
+	} else if (shdr_rel_plt.sh_link == ss->symtab_idx) {
+		/*
+		 * A static executable can have a .plt due to IFUNCs, in which
+		 * case .symtab is used not .dynsym.
+		 */
+		scn_dynsym = ss->symtab;
+		shdr_dynsym = ss->symshdr;
+	} else {
 		goto out_elf_end;
+	}
+
+	if (!scn_dynsym)
+		return 0;
 
 	/*
 	 * Fetch the relocation section to find the idxes to the GOT
···
 
 	ss->is_64_bit = (gelf_getclass(elf) == ELFCLASS64);
 
+	ss->symtab_idx = 0;
 	ss->symtab = elf_section_by_name(elf, &ehdr, &ss->symshdr, ".symtab",
-			NULL);
+			&ss->symtab_idx);
 	if (ss->symshdr.sh_type != SHT_SYMTAB)
 		ss->symtab = NULL;
 

tools/perf/util/symsrc.h

···
 	GElf_Shdr	     opdshdr;
 
 	Elf_Scn		     *symtab;
+	size_t		     symtab_idx;
 	GElf_Shdr	     symshdr;
 
 	Elf_Scn		     *dynsym;