linux-kernel - [PATCH v2] perf tools: don't adjust symbols in vDSO

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Date:	Sat, 27 Jun 2015 10:10:13 +0100
From:	Will Deacon <will.deacon@....com>
To:	acme@...hat.com
Cc:	linux-kernel@...r.kernel.org, kristina.martsenko@....com,
	Will Deacon <will.deacon@....com>,
	Vladimir Nikulichev <nvs@...icks.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Andy Lutomirski <luto@...capital.net>
Subject: [PATCH v2] perf tools: don't adjust symbols in vDSO

Commit 922d0e4d9f04 ("perf tools: Adjust symbols in VDSO") changed the
ELF symbol parsing so that the vDSO is treated the same as ET_EXEC and
ET_REL binaries despite being an ET_DYN. This was a partial workaround
to deal with older x86 vDSOs being prelinked at a high address that
didn't correspond to the map, so using object-relative offsets and
adding the base of the map allowed symbol resolution to succeed.

Unfortunately, this causes objdump not to produce any output in
conjunction with perf annotate, which cheerfully passes the absolute
address of the map symbol.

This patch fixes the problem by avoiding adjustment of vDSO symbols and
instead setting the map->pgoff field to correspond to the virtual load
address specified in the vDSO ELF header.

Cc: Vladimir Nikulichev <nvs@...icks.com>
Cc: Adrian Hunter <adrian.hunter@...el.com>
Cc: Namhyung Kim <namhyung@...nel.org>
Cc: Andy Lutomirski <luto@...capital.net>
Reported-by: Kristina Martsenko <kristina.martsenko@....com>
Signed-off-by: Will Deacon <will.deacon@....com>
---

v1->v2: Adjust map->pgoff in ELF loader to avoid breaking symbol lookup
        on older kernels.

 tools/perf/util/map.c        | 5 ++---
 tools/perf/util/symbol-elf.c | 9 ++++++++-
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/tools/perf/util/map.c b/tools/perf/util/map.c
index a14f08f41686..6ba38293be88 100644
--- a/tools/perf/util/map.c
+++ b/tools/perf/util/map.c
@@ -173,10 +173,9 @@ struct map *map__new(struct machine *machine, u64 start, u64 len,
 				filename = newfilename;
 		}
 
-		if (vdso) {
-			pgoff = 0;
+		if (vdso)
 			dso = vdso__dso_findnew(machine, thread);
-		} else
+		else
 			dso = __dsos__findnew(&machine->user_dsos, filename);
 
 		if (dso == NULL)
diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index a7ab6063e038..83f8ba232575 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -706,7 +706,6 @@ int symsrc__init(struct symsrc *ss, struct dso *dso, const char *name,
 		GElf_Shdr shdr;
 		ss->adjust_symbols = (ehdr.e_type == ET_EXEC ||
 				ehdr.e_type == ET_REL ||
-				dso__is_vdso(dso) ||
 				elf_section_by_name(elf, &ehdr, &shdr,
 						     ".gnu.prelink_undo",
 						     NULL) != NULL);
@@ -824,6 +823,14 @@ int dso__load_sym(struct dso *dso, struct map *map,
 	sec = syms_ss->symtab;
 	shdr = syms_ss->symshdr;
 
+	/*
+	 * Older x86 kernels prelink the vDSO at a high address, so
+	 * we need to reflect that in map->pgoff in order to talk to
+	 * objdump.
+	 */
+	if (dso__is_vdso(dso))
+		map->pgoff = shdr.sh_addr - shdr.sh_offset;
+
 	if (runtime_ss->opdsec)
 		opddata = elf_rawdata(runtime_ss->opdsec, NULL);
 
-- 
2.1.4

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