[tip: objtool/core] objtool/klp: Fix size of empty special section entries

[tip-bot2 for Josh Poimboeuf]

The following commit has been merged into the objtool/core branch of tip:

Commit-ID:     1ea786097cd79522b76cbd41beeb8f84ef3a76f4
Gitweb:        https://git.kernel.org/tip/1ea786097cd79522b76cbd41beeb8f84ef3a76f4
Author:        Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
AuthorDate:    Fri, 07 Aug 2026 14:37:47 -07:00
Committer:     Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
CommitterDate: Tue, 11 Aug 2026 15:04:57 -07:00

objtool/klp: Fix size of empty special section entries

create_fake_symbols() sizes each ANNOTATE_DATA_SPECIAL entry from the
offset of the next annotation, falling back to the end of the section
for the last entry.  But the last entry is detected by a zero size,
which also happens for an *empty* entry: ALTERNATIVE(oldinstr, "", ft)
still annotates its zero-length replacement, at the same offset as the
next entry's annotation.

So every empty replacement gets a fake symbol spanning the entire rest
of .altinstr_replacement.  That's harmless today only because
find_symbol_containing() picks the smaller of two overlapping symbols.

Track whether a next annotation was found rather than inferring it from
the size.  A zero-length fake symbol is fine: find_symbol_containing()
skips those, so the properly sized symbol at the same offset still wins.

Fixes: dd590d4d57eb ("objtool/klp: Introduce klp diff subcommand for diffing object files")
Acked-by: Song Liu <song@xxxxxxxxxx>
Acked-by: Joe Lawrence <joe.lawrence@xxxxxxxxxx>
Link: https://patch.msgid.link/913e691c5009397df832c7c9a18cd5cf71b42737.1786138493.git.jpoimboe@xxxxxxxxxx
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 tools/objtool/klp-diff.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/tools/objtool/klp-diff.c b/tools/objtool/klp-diff.c
index c5284d2..b4220e1 100644
--- a/tools/objtool/klp-diff.c
+++ b/tools/objtool/klp-diff.c
@@ -1628,13 +1628,17 @@ static int create_fake_symbols(struct elf *elf)
 	for_each_reloc(sec->rsec, reloc) {
 		unsigned long offset, size;
 		struct reloc *next_reloc;
+		bool last = true;
 
 		if (annotype(elf, sec, reloc) != ANNOTYPE_DATA_SPECIAL)
 			continue;
 
 		offset = reloc_addend(reloc);
 
-		size = 0;
+		/*
+		 * Find the start of the next entry so the fake symbol size can
+		 * be calculated.
+		 */
 		next_reloc = reloc;
 		for_each_reloc_continue(sec->rsec, next_reloc) {
 			if (annotype(elf, sec, next_reloc) != ANNOTYPE_DATA_SPECIAL ||
@@ -1642,10 +1646,15 @@ static int create_fake_symbols(struct elf *elf)
 				continue;
 
 			size = reloc_addend(next_reloc) - offset;
+			last = false;
 			break;
 		}
 
-		if (!size)
+		/*
+		 * If no next entry found, this is the last entry, so its size
+		 * is from the current offset to the end of the section.
+		 */
+		if (last)
 			size = sec_size(reloc->sym->sec) - offset;
 
 		if (create_fake_symbol(elf, reloc->sym->sec, offset, size))