linux-kernel - [PATCH 2/2] perf symbols: work around incorrect ET

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Date:	Tue,  3 Aug 2010 12:48:36 +0100
From:	Dave Martin <dave.martin@...aro.org>
To:	linux-kernel@...r.kernel.org
Cc:	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	kernel-team@...ts.ubuntu.com, Will Deacon <Will.Deacon@....com>,
	Linaro Dev <linaro-dev@...ts.linaro.org>,
	Dave Martin <dave.martin@...aro.org>
Subject: [PATCH 2/2] perf symbols: work around incorrect ET_EXEC symbol adjustment

Workaround to adjust .text symbols from ET_EXEC images correctly.

This is not a complete fix, but addresses the common case and does not
create additional assumptions beyond those which perf already makes.
ELF images with a weird link map may still be processed incorrectly
(as at present) -- that will require a separate fix.

The workaround allows correct symbol offsets to be generated for
ET_EXEC executables on systems with separated debug info (where
section sh_offset fields from the debug image may bear no relation to
the layout of the executable image).

The cover note accompanying this patch contains a more detailed
explanation.

Signed-off-by: Dave Martin <dave.martin@...aro.org>
---
 tools/perf/util/symbol.c |   10 +++++++++-
 1 files changed, 9 insertions(+), 1 deletions(-)

diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
index 3b8c005..3930398 100644
--- a/tools/perf/util/symbol.c
+++ b/tools/perf/util/symbol.c
@@ -1135,7 +1135,15 @@ static int dso__load_sym(struct dso *self, struct map *map, const char *name,
 				  "sh_addr: %#Lx sh_offset: %#Lx\n", __func__,
 				  (u64)sym.st_value, (u64)shdr.sh_addr,
 				  (u64)shdr.sh_offset);
-			sym.st_value -= shdr.sh_addr - shdr.sh_offset;
+			/* Assumptions:
+			 *	a) shdr.sh_addr - shdr.sh_offset ==
+			 *		map->start - map->pgoff
+			 *	b) map->pgoff == 0
+			 * These are true iff we are looking at a function
+			 * symbol in the main executable segment _and_
+			 * the main executable segment starts at the start of
+			 * the ELF image (normally true). */
+			sym.st_value -= map->start;
 		}
 		/*
 		 * We need to figure out if the object was created from C++ sources
-- 
1.7.0.4

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