[RFC PATCH v2 3/7] 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.

Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
---
 tools/objtool/klp-diff.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/tools/objtool/klp-diff.c b/tools/objtool/klp-diff.c
index f40c128fdd49..4195b3ccff15 100644
--- a/tools/objtool/klp-diff.c
+++ b/tools/objtool/klp-diff.c
@@ -21,6 +21,8 @@
 #include <linux/string.h>
 #include <linux/jhash.h>
 
+#include <arch/special.h>
+
 #define sizeof_field(TYPE, MEMBER) sizeof((((TYPE *)0)->MEMBER))
 
 struct elfs {
@@ -1638,6 +1640,22 @@ 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)
 {
@@ -1781,6 +1799,27 @@ 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 && sec_size(sec) % entry_size) {
+				ERROR("%s: entry size %u doesn't divide section size %lu",
+				      sec->name, entry_size, sec_size(sec));
+				return -1;
+			}
+		}
+
 		if (!entry_size) {
 			entry_size = arch_reloc_size(sec->rsec->relocs);
 			if (sec_size(sec) != entry_size * sec_num_entries(sec->rsec)) {
-- 
2.55.0