linux-kernel - [PATCH 22/22] objtool: Skip reading DWARF section data

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Message-Id: <52a9698835861dd35f2ec35c49f96d0bb39fb177.1685464332.git.jpoimboe@kernel.org>
Date:   Tue, 30 May 2023 10:21:14 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     x86@...nel.org
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Miroslav Benes <mbenes@...e.cz>
Subject: [PATCH 22/22] objtool: Skip reading DWARF section data

Objtool doesn't use DWARF at all, and the DWARF sections' data take up a
lot of memory.  Skip reading them.

Note this only skips the DWARF base sections, not the rela sections.
The relas are needed because their symbol references may need to be
reindexed if any local symbols get added by elf_create_symbol().

Also note the DWARF data will eventually be read by libelf anyway, when
writing the object file.  But that's fine, the goal here is to reduce
*peak* memory usage, and the previous patch (which freed insn memory)
gave some breathing room.  So the allocation gets shifted to a later
time, resulting in lower peak memory usage.

With allyesconfig + CONFIG_DEBUG_INFO:

- Before: peak heap memory consumption: 29.93G
- After:  peak heap memory consumption: 25.47G

Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
---
 tools/objtool/elf.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 54d182ddc4bb..d420b5d2e2b6 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -300,6 +300,11 @@ struct reloc *find_reloc_by_dest(const struct elf *elf, struct section *sec, uns
 	return find_reloc_by_dest_range(elf, sec, offset, 1);
 }

+static bool is_dwarf_section(struct section *sec)
+{
+	return !strncmp(sec->name, ".debug_", 7);
+}
+
 static int read_sections(struct elf *elf)
 {
 	Elf_Scn *s = NULL;
@@ -350,7 +355,7 @@ static int read_sections(struct elf *elf)
 			return -1;
 		}

-		if (sec->sh.sh_size != 0) {
+		if (sec->sh.sh_size != 0 && !is_dwarf_section(sec)) {
 			sec->data = elf_getdata(s, NULL);
 			if (!sec->data) {
 				WARN_ELF("elf_getdata");
-- 
2.40.1