[RFC 2/4] objtool/klp: allow special section entry size overrides

[Joe Lawrence <joe.lawrence@redhat.com>]

Special section entry sizes (ALT_ENTRY_SIZE, JUMP_ENTRY_SIZE,
EX_ENTRY_SIZE) are built into objtool from arch-specific headers.
When processing cached unit test objects that were built from a
different kernel version, these compiled-in sizes may not match the
objects' actual entry sizes, causing create_fake_symbols() to
incorrectly parse special sections.

Allow the user to override the compiled-in defaults via environment
variables of the same name.  When unset, behavior is unchanged.  This
will enable a klp-diff unit test runner to pass the correct entry sizes
from test metadata.

Assisted-by: Cursor:claude-4.6-opus
Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
---
 tools/objtool/klp-diff.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/tools/objtool/klp-diff.c b/tools/objtool/klp-diff.c
index bd8d64f2f3f6..ebe4a2a087ca 100644
--- a/tools/objtool/klp-diff.c
+++ b/tools/objtool/klp-diff.c
@@ -1749,6 +1749,22 @@ static int clone_sym_relocs(struct elfs *e, struct symbol *patched_sym)
 
 }
 
+static unsigned int entry_size_from_env(const char *name, unsigned int def)
+{
+	const char *str = getenv(name);
+	char *end;
+	unsigned long val;
+
+	if (!str)
+		return def;
+
+	val = strtoul(str, &end, 10);
+	if (*end || !val)
+		return def;
+
+	return val;
+}
+
 static int create_fake_symbol(struct elf *elf, struct section *sec,
 			      unsigned long offset, size_t size)
 {
@@ -1871,6 +1887,21 @@ static int create_fake_symbols(struct elf *elf)
 		}
 
 		entry_size = sec->sh.sh_entsize;
+
+		/*
+		 * Some special sections have multiple relocs per entry,
+		 * so the reloc-based heuristic below doesn't work.  Use
+		 * the arch-defined entry sizes for known special sections.
+		 */
+		if (!entry_size) {
+			if (!strcmp(sec->name, ".altinstructions"))
+				entry_size = entry_size_from_env("ALT_ENTRY_SIZE", ALT_ENTRY_SIZE);
+			else if (!strcmp(sec->name, "__jump_table"))
+				entry_size = entry_size_from_env("JUMP_ENTRY_SIZE", JUMP_ENTRY_SIZE);
+			else if (!strcmp(sec->name, "__ex_table"))
+				entry_size = entry_size_from_env("EX_ENTRY_SIZE", EX_ENTRY_SIZE);
+		}
+
 		if (!entry_size) {
 			entry_size = arch_reloc_size(sec->rsec->relocs);
 			if (sec_size(sec) != entry_size * sec_num_entries(sec->rsec)) {
-- 
2.53.0