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Subject[tip: x86/core] objtool: Ignore extra-symbol code
The following commit has been merged into the x86/core branch of tip:

Commit-ID: 4adb23686795e9c88e3217b5d7b4524c0da9d04f
Gitweb: https://git.kernel.org/tip/4adb23686795e9c88e3217b5d7b4524c0da9d04f
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Tue, 08 Mar 2022 16:30:46 +01:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 15 Mar 2022 10:32:43 +01:00

objtool: Ignore extra-symbol code

There's a fun implementation detail on linking STB_WEAK symbols. When
the linker combines two translation units, where one contains a weak
function and the other an override for it. It simply strips the
STB_WEAK symbol from the symbol table, but doesn't actually remove the
code.

The result is that when objtool is ran in a whole-archive kind of way,
it will encounter *heaps* of unused (and unreferenced) code. All
rudiments of weak functions.

Additionally, when a weak implementation is split into a .cold
subfunction that .cold symbol is left in place, even though completely
unused.

Teach objtool to ignore such rudiments by searching for symbol holes;
that is, code ranges that fall outside the given symbol bounds.
Specifically, ignore a sequence of unreachable instruction iff they
occupy a single hole, additionally ignore any .cold subfunctions
referenced.

Both ld.bfd and ld.lld behave like this. LTO builds otoh can (and do)
properly DCE weak functions.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20220308154319.232019347@infradead.org
---
tools/objtool/check.c | 43 ++++++++++++++++++++-
tools/objtool/elf.c | 60 ++++++++++++++++++++++++++++-
tools/objtool/include/objtool/elf.h | 1 +-
3 files changed, 104 insertions(+)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index ae1d4f9..0e0e5b5 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -3346,6 +3346,49 @@ static bool ignore_unreachable_insn(struct objtool_file *file, struct instructio
!strcmp(insn->sec->name, ".altinstr_aux"))
return true;

+ /*
+ * Whole archive runs might encounder dead code from weak symbols.
+ * This is where the linker will have dropped the weak symbol in
+ * favour of a regular symbol, but leaves the code in place.
+ *
+ * In this case we'll find a piece of code (whole function) that is not
+ * covered by a !section symbol. Ignore them.
+ */
+ if (!insn->func && lto) {
+ int size = find_symbol_hole_containing(insn->sec, insn->offset);
+ unsigned long end = insn->offset + size;
+
+ if (!size) /* not a hole */
+ return false;
+
+ if (size < 0) /* hole until the end */
+ return true;
+
+ sec_for_each_insn_continue(file, insn) {
+ /*
+ * If we reach a visited instruction at or before the
+ * end of the hole, ignore the unreachable.
+ */
+ if (insn->visited)
+ return true;
+
+ if (insn->offset >= end)
+ break;
+
+ /*
+ * If this hole jumps to a .cold function, mark it ignore too.
+ */
+ if (insn->jump_dest && insn->jump_dest->func &&
+ strstr(insn->jump_dest->func->name, ".cold")) {
+ struct instruction *dest = insn->jump_dest;
+ func_for_each_insn(file, dest->func, dest)
+ dest->ignore = true;
+ }
+ }
+
+ return false;
+ }
+
if (!insn->func)
return false;

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 456ac22..d7b99a7 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -83,6 +83,31 @@ static int symbol_by_offset(const void *key, const struct rb_node *node)
return 0;
}

+struct symbol_hole {
+ unsigned long key;
+ const struct symbol *sym;
+};
+
+/*
+ * Find !section symbol where @offset is after it.
+ */
+static int symbol_hole_by_offset(const void *key, const struct rb_node *node)
+{
+ const struct symbol *s = rb_entry(node, struct symbol, node);
+ struct symbol_hole *sh = (void *)key;
+
+ if (sh->key < s->offset)
+ return -1;
+
+ if (sh->key >= s->offset + s->len) {
+ if (s->type != STT_SECTION)
+ sh->sym = s;
+ return 1;
+ }
+
+ return 0;
+}
+
struct section *find_section_by_name(const struct elf *elf, const char *name)
{
struct section *sec;
@@ -162,6 +187,41 @@ struct symbol *find_symbol_containing(const struct section *sec, unsigned long o
return NULL;
}

+/*
+ * Returns size of hole starting at @offset.
+ */
+int find_symbol_hole_containing(const struct section *sec, unsigned long offset)
+{
+ struct symbol_hole hole = {
+ .key = offset,
+ .sym = NULL,
+ };
+ struct rb_node *n;
+ struct symbol *s;
+
+ /*
+ * Find the rightmost symbol for which @offset is after it.
+ */
+ n = rb_find(&hole, &sec->symbol_tree, symbol_hole_by_offset);
+
+ /* found a symbol that contains @offset */
+ if (n)
+ return 0; /* not a hole */
+
+ /* didn't find a symbol for which @offset is after it */
+ if (!hole.sym)
+ return 0; /* not a hole */
+
+ /* @offset >= sym->offset + sym->len, find symbol after it */
+ n = rb_next(&hole.sym->node);
+ if (!n)
+ return -1; /* until end of address space */
+
+ /* hole until start of next symbol */
+ s = rb_entry(n, struct symbol, node);
+ return s->offset - offset;
+}
+
struct symbol *find_func_containing(struct section *sec, unsigned long offset)
{
struct rb_node *node;
diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
index d223367..22ba7e2 100644
--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -152,6 +152,7 @@ struct symbol *find_func_by_offset(struct section *sec, unsigned long offset);
struct symbol *find_symbol_by_offset(struct section *sec, unsigned long offset);
struct symbol *find_symbol_by_name(const struct elf *elf, const char *name);
struct symbol *find_symbol_containing(const struct section *sec, unsigned long offset);
+int find_symbol_hole_containing(const struct section *sec, unsigned long offset);
struct reloc *find_reloc_by_dest(const struct elf *elf, struct section *sec, unsigned long offset);
struct reloc *find_reloc_by_dest_range(const struct elf *elf, struct section *sec,
unsigned long offset, unsigned int len);
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