create-diff-object: Fix ordering of __patchable_function_entries

Old linkers don't support mixing ordered and unordered sections. This
was fixed in binutils, see discussion in [1]. But without this fix,
kpatch-build fails with an error like:

| ld.bfd: __patchable_function_entries has both ordered
| [`__patchable_function_entries' in /builddir/.kpatch/tmp/patch/output.o]
| and unordered [`__patchable_function_entries' in
| /builddir/.kpatch/tmp/patch/patch-hook.o] sections
| ld.bfd: final link failed: bad value

Fix this by only setting SHF_LINK_ORDER in the output object if the
patched object also has this flag set.

Signed-off-by: Puranjay Mohan <pjy@amazon.com>

Author: puranjaymohan

kpatch-build/create-diff-object.c

@@ -1992,6 +1992,8 @@ static void kpatch_migrate_included_elements(struct kpatch_elf *kelf, struct kpa
 		ERROR("malloc");
 	memset(out, 0, sizeof(*out));
 	out->arch = kelf->arch;
+	out->has_pfe = kelf->has_pfe;
+	out->pfe_ordered = kelf->pfe_ordered;
 	INIT_LIST_HEAD(&out->sections);
 	INIT_LIST_HEAD(&out->symbols);
 	INIT_LIST_HEAD(&out->strings);
@@ -3729,7 +3731,7 @@ static void kpatch_set_pfe_link(struct kpatch_elf *kelf)
  * TODO: Eventually we can modify recordmount so that it recognizes our bundled
  * sections as valid and does this work for us.
  */
-static void kpatch_create_ftrace_callsite_sections(struct kpatch_elf *kelf, bool has_pfe)
+static void kpatch_create_ftrace_callsite_sections(struct kpatch_elf *kelf)
 {
 	int nr, index;
 	struct section *sec = NULL;
@@ -3745,7 +3747,7 @@ static void kpatch_create_ftrace_callsite_sections(struct kpatch_elf *kelf, bool
 		    sym->has_func_profiling)
 			nr++;
 
-	if (has_pfe)
+	if (kelf->has_pfe)
 		/*
 		 * Create separate __patchable_function_entries sections
 		 * for each function in the following loop.
@@ -3858,7 +3860,14 @@ static void kpatch_create_ftrace_callsite_sections(struct kpatch_elf *kelf, bool
 			 *   - its lone rela is based on the section symbol
 			 */
 			sec = create_section_pair(kelf, "__patchable_function_entries", sizeof(void *), 1);
-			sec->sh.sh_flags |= SHF_WRITE | SHF_ALLOC | SHF_LINK_ORDER;
+			sec->sh.sh_flags |= SHF_WRITE | SHF_ALLOC;
+			/*
+			 * Old linkers don't support mixing ordered and unordered sections, so set
+			 * the SHF_LINK_ORDER flag only if it is set in the pfe section generated by
+			 * the compiler.
+			 */
+			if (kelf->pfe_ordered)
+				sec->sh.sh_flags |= SHF_LINK_ORDER;
 			rela_sym = sym->sec->secsym;
 			rela_offset = 0;
 			rela_sym->pfe = sec;
@@ -4158,7 +4167,6 @@ int main(int argc, char *argv[])
 	struct section *relasec, *symtab;
 	char *orig_obj, *patched_obj, *parent_name;
 	char *parent_symtab, *mod_symvers, *patch_name, *output_obj;
-	bool has_pfe = false;
 
 	memset(&arguments, 0, sizeof(arguments));
 	argp_parse (&argp, argc, argv, 0, NULL, &arguments);
@@ -4184,8 +4192,6 @@ int main(int argc, char *argv[])
 
 	kpatch_set_pfe_link(kelf_orig);
 	kpatch_set_pfe_link(kelf_patched);
-	if (kelf_patched->has_pfe)
-		has_pfe = true;
 
 	kpatch_find_func_profiling_calls(kelf_orig);
 	kpatch_find_func_profiling_calls(kelf_patched);
@@ -4266,7 +4272,7 @@ int main(int argc, char *argv[])
 	kpatch_create_callbacks_objname_rela(kelf_out, parent_name);
 	kpatch_build_strings_section_data(kelf_out);
 
-	kpatch_create_ftrace_callsite_sections(kelf_out, has_pfe);
+	kpatch_create_ftrace_callsite_sections(kelf_out);
 
 	/*
 	 *  At this point, the set of output sections and symbols is

kpatch-build/kpatch-elf.c

@@ -559,6 +559,7 @@ struct kpatch_elf *kpatch_elf_open(const char *name)
 	int fd;
 	struct kpatch_elf *kelf;
 	struct section *relasec;
+	struct section *pfesec;
 	GElf_Ehdr ehdr;
 
 	fd = open(name, O_RDONLY);
@@ -614,9 +615,17 @@ struct kpatch_elf *kpatch_elf_open(const char *name)
 	 * These sections aren't used by ftrace on this arch, so do not
 	 * bother reading/writing them for x86_64.
 	 */
-	if (kelf->arch != X86_64)
-		if (find_section_by_name(&kelf->sections, "__patchable_function_entries"))
+	if (kelf->arch != X86_64) {
+		pfesec = find_section_by_name(&kelf->sections, "__patchable_function_entries");
+		if (pfesec) {
 			kelf->has_pfe = true;
+			/*
+			 * Check if the pfe section generated by the toolchain has the SHF_LINK_ORDER
+			 * flag set. This info is used while generating pfe sections in the diff object.
+			 */
+			kelf->pfe_ordered = !!(pfesec->sh.sh_flags & SHF_LINK_ORDER);
+		}
+	}
 
 	return kelf;
 }

kpatch-build/kpatch-elf.h

@@ -127,6 +127,7 @@ struct kpatch_elf {
 	Elf_Data *symtab_shndx;
 	int fd;
 	bool has_pfe;
+	bool pfe_ordered;
 };
 
 /*******************