perf symbol: Skip symbols if SHF_ALLOC flag is not set

Some symbols are observed with the 'st_value' field zeroed.  E.g.
libc.so.6 in Ubuntu contains a symbol '__evoke_link_warning_getwd' which
resides in the '.gnu.warning.getwd' section.

Unlike normal sections, such kind of sections are used for linker
warning when a file calls deprecated functions, but they are not part of
memory images, the symbols in these sections should be dropped.

This patch checks the section attribute SHF_ALLOC bit, if the bit is not
set, it skips symbols to avoid spurious ones.

Suggested-by: Fangrui Song <[email protected]>
Signed-off-by: Leo Yan <[email protected]>
Acked-by: Namhyung Kim <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Chang Rui <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: Leo Yan

tools/perf/util/symbol-elf.c

@@ -1255,6 +1255,17 @@ dso__load_sym_internal(struct dso *dso, struct map *map, struct symsrc *syms_ss,
 
 		gelf_getshdr(sec, &shdr);
 
+		/*
+		 * If the attribute bit SHF_ALLOC is not set, the section
+		 * doesn't occupy memory during process execution.
+		 * E.g. ".gnu.warning.*" section is used by linker to generate
+		 * warnings when calling deprecated functions, the symbols in
+		 * the section aren't loaded to memory during process execution,
+		 * so skip them.
+		 */
+		if (!(shdr.sh_flags & SHF_ALLOC))
+			continue;
+
 		secstrs = secstrs_sym;
 
 		/*