[PATCH v3 0/9] objtool/klp: sympos/module/alternative/etc fixes

[PATCH v3 0/9] objtool/klp: sympos/module/alternative/etc fixes

[Josh Poimboeuf]

v3:
- remove "size = 0" and add comments to create_fake_symbols()
- add patches 8 & 9

v2: https://lore.kernel.org/cover.1785939903.git.jpoimboe@kernel.org
- rebased on tip/master (first 6 patches were merged)
- dropped original patches 7-8 (will post followup)
- added .klp.symid fix

v1: https://lore.kernel.org/cover.1785727106.git.jpoimboe@kernel.org

This consolidates fixes for the klp-build issues reported by Joe over
the last several weeks, plus some more things I found while
testing/reviewing.

Joe Lawrence (1):
  objtool/klp: Allow new references to module exports

Josh Poimboeuf (8):
  objtool/klp: Fix vmlinux .klp.symid link error for .no_trim_symbol
    symbols
  objtool/klp: Fix size of empty special section entries
  objtool/klp: Ignore replacement offset of empty x86 alternatives
  objtool/klp: Explicitly disallow patching or referencing init
    code/data
  objtool/klp: Fix cross-module klp relocation section naming
  objtool/klp: Don't match local symbols against exports
  objtool/klp: Fix relocations for EXPORT_SYMBOL_FOR_MODULES() symbols
  objtool/klp: Fix line numbers in Module.symvers parse errors

 tools/objtool/arch/x86/special.c        |  27 ++++++
 tools/objtool/include/objtool/klp.h     |  10 ++-
 tools/objtool/include/objtool/special.h |   7 ++
 tools/objtool/klp-diff.c                | 106 ++++++++++++++++++++----
 tools/objtool/klp-post-link.c           |  53 +++++++-----
 tools/objtool/klp-symid.c               |   1 +
 tools/objtool/klp-sympos.c              |  10 +++
 7 files changed, 175 insertions(+), 39 deletions(-)

-- 
2.54.0

[PATCH v3 1/9] objtool/klp: Fix vmlinux .klp.symid link error for .no_trim_symbol symbols

[Josh Poimboeuf]

Testing klp-build with arm64 produced the following linker error during
the original kernel build:

  `__notrim.1' referenced in section `.klp.symid' of vmlinux.o: defined in discarded section `.no_trim_symbol' of vmlinux.o

symbol_get() puts a static __notrim[] in .no_trim_symbol, which GCC
names __notrim.1, __notrim.2, etc.  Two or more built-in translation
units calling symbol_get() thus produce duplicate names, resulting in
corresponding .klp.symid references which trigger the above error.

Add .no_trim_symbol to the discarded section list so its symbols don't
get symids.

Note this issue is not specific to arm64: it just needs two built-in
symbol_get() callers.  arm64 trips over it easily because it has KVM
always compiled in vmlinux, whereas on x86 it's typically a module.

Fixes: 029223d30162 ("objtool/klp: Add .klp.symid for sympos disambiguation")
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 tools/objtool/klp-symid.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/objtool/klp-symid.c b/tools/objtool/klp-symid.c
index cf188cdfa6079..21d8708013aba 100644
--- a/tools/objtool/klp-symid.c
+++ b/tools/objtool/klp-symid.c
@@ -31,6 +31,7 @@
 static const char * const discarded_secs[] = {
 	".discard",
 	".modinfo",
+	".no_trim_symbol",
 	"__tracepoint_check",
 };
 
-- 
2.54.0

[PATCH v3 2/9] objtool/klp: Fix size of empty special section entries

[Josh Poimboeuf]

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@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 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 492d7a012cffe..38fae861d12c7 100644
--- a/tools/objtool/klp-diff.c
+++ b/tools/objtool/klp-diff.c
@@ -1627,13 +1627,17 @@ 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 ||
@@ -1641,10 +1645,15 @@ 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))
-- 
2.54.0

[PATCH v3 3/9] objtool/klp: Ignore replacement offset of empty x86 alternatives

[Josh Poimboeuf]

An x86 alternative with an empty replacement, e.g. the second entry of

  ALTERNATIVE_2("orig", "repl", ft1, "", ft2)

has a replacementlen of zero.  Its replacement offset still gets a
relocation, but the label it points at is the end of the previous
replacement, which is also the beginning of the *next* alternative's
replacement.  The value is meaningless; get_alt_entry() already ignores
it for that reason.

klp diff doesn't ignore it.  When such an alternative belongs to a
changed function, cloning its relocations drags in the unrelated
neighboring replacement, along with everything that replacement
references.  On an x86 clang/lto build an empty alternative in
meminfo_proc_show() pulled in the replacement of an alternative in
proc_kcore_init(), silently emitting a klp relocation against init text
which has long since been freed by the time the patch is applied.

Add arch_alt_ignore_new_reloc() and skip such relocations when cloning.
This has to be arch specific: on arm64 a zero-length replacement instead
identifies an alternative callback, whose replacement offset points at
the callback function and must be preserved.

Fixes: dd590d4d57eb ("objtool/klp: Introduce klp diff subcommand for diffing object files")
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 tools/objtool/arch/x86/special.c        | 27 +++++++++++++++++++++++++
 tools/objtool/include/objtool/special.h |  7 +++++++
 tools/objtool/klp-diff.c                |  6 +++++-
 3 files changed, 39 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/arch/x86/special.c b/tools/objtool/arch/x86/special.c
index e817a3fff4491..1e84c81bfcd81 100644
--- a/tools/objtool/arch/x86/special.c
+++ b/tools/objtool/arch/x86/special.c
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
 #include <string.h>
 
+#include <arch/special.h>
 #include <objtool/special.h>
 #include <objtool/builtin.h>
 #include <objtool/warn.h>
@@ -9,6 +10,32 @@
 /* cpu feature name array generated from cpufeatures.h */
 #include "cpu-feature-names.c"
 
+/*
+ * An alternative with an empty replacement, e.g. the second entry of
+ *
+ *   ALTERNATIVE_2("orig", "repl", ft1, "", ft2)
+ *
+ * still gets a relocation for its replacement offset.  But the label it points
+ * at is the end of the previous entry's replacement, which is also the
+ * beginning of the *next* entry's replacement.  The value is meaningless: it's
+ * only ever used with a length of zero.
+ */
+bool arch_alt_ignore_new_reloc(struct section *sec, unsigned long offset)
+{
+	unsigned long entry_off;
+
+	if (strcmp(sec->name, ".altinstructions"))
+		return false;
+
+	entry_off = offset - (offset % ALT_ENTRY_SIZE);
+
+	if (offset - entry_off != ALT_NEW_OFFSET)
+		return false;
+
+	return !*(unsigned char *)(sec->data->d_buf + entry_off +
+				   ALT_NEW_LEN_OFFSET);
+}
+
 void arch_handle_alternative(struct special_alt *alt)
 {
 	static struct special_alt *group, *prev;
diff --git a/tools/objtool/include/objtool/special.h b/tools/objtool/include/objtool/special.h
index 121c3761899c1..620dbf6cb0e58 100644
--- a/tools/objtool/include/objtool/special.h
+++ b/tools/objtool/include/objtool/special.h
@@ -32,6 +32,13 @@ int special_get_alts(struct elf *elf, struct list_head *alts);
 
 void arch_handle_alternative(struct special_alt *alt);
 
+/*
+ * Should the reloc at @offset -- the "new" (replacement) field of a special
+ * section group entry -- be ignored?  The meaning of a zero-length replacement
+ * is arch specific, so the arch decides.
+ */
+bool arch_alt_ignore_new_reloc(struct section *sec, unsigned long offset);
+
 bool arch_support_alt_relocation(struct special_alt *special_alt,
 				 struct instruction *insn,
 				 struct reloc *reloc);
diff --git a/tools/objtool/klp-diff.c b/tools/objtool/klp-diff.c
index 38fae861d12c7..3923fabc13331 100644
--- a/tools/objtool/klp-diff.c
+++ b/tools/objtool/klp-diff.c
@@ -12,7 +12,7 @@
 #include <objtool/arch.h>
 #include <objtool/klp.h>
 #include <objtool/util.h>
-#include <arch/special.h>
+#include <objtool/special.h>
 
 #include <linux/align.h>
 #include <linux/objtool_types.h>
@@ -1537,6 +1537,10 @@ static int clone_sym_relocs(struct elfs *e, struct symbol *patched_sym)
 		    !strcmp(patched_reloc->sym->sec->name, ".altinstr_aux"))
 			continue;
 
+		if (arch_alt_ignore_new_reloc(patched_sym->sec,
+					      reloc_offset(patched_reloc)))
+			continue;
+
 		ret = convert_reloc_sym(e->patched, patched_reloc);
 		if (ret < 0) {
 			ERROR_FUNC(patched_rsec->base, reloc_offset(patched_reloc),
-- 
2.54.0

[PATCH v3 4/9] objtool/klp: Explicitly disallow patching or referencing init code/data

[Josh Poimboeuf]

Explicitly disallow the patching and referencing of init code/data.
Otherwise it could potentially introduce some odd edge cases depending
on whether the target object's init section has been freed yet (note
that the init code still exists in the target module when doing late
module patching).

Such edge cases include sympos calculation and the patching and/or
referencing of non-existent (init-freed) code/data.  Not to mention the
inherent differences in behavior that occur when the init code is only
patched *some* of the time depending on module loading order or kernel
config.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 tools/objtool/klp-sympos.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/tools/objtool/klp-sympos.c b/tools/objtool/klp-sympos.c
index bbfae516d3395..dfca9dd746812 100644
--- a/tools/objtool/klp-sympos.c
+++ b/tools/objtool/klp-sympos.c
@@ -367,6 +367,11 @@ static unsigned long find_vmlinux_sympos(struct symbol *sym)
 	return sympos;
 }
 
+static bool is_init_sym(struct symbol *sym)
+{
+	return strstarts(sym->sec->name, ".init");
+}
+
 /*
  * "sympos" is used by livepatch to disambiguate duplicate symbol names.
  */
@@ -376,6 +381,11 @@ unsigned long klp_find_sympos(struct elf *elf, struct symbol *sym)
 	bool has_dup = false;
 	struct symbol *s;
 
+	if (is_init_sym(sym)) {
+		ERROR("%s: can't patch or reference init code/data", sym->name);
+		return ULONG_MAX;
+	}
+
 	if (sym->bind != STB_LOCAL)
 		return 0;
 
-- 
2.54.0

[PATCH v3 5/9] objtool/klp: Fix cross-module klp relocation section naming

[Josh Poimboeuf]

A klp relocation section is .klp.rela.<objname>.<secname>, where objname
is the object being patched.

klp-build wrongly derives objname from where the referenced symbol
lives, not where it's referenced.  For a cross-module reference like
patched can_isotp code calling can.ko's can_rx_unregister(), that gives
.klp.rela.can..text rather than .klp.rela.can_isotp..text.  Unless the
patch happens to patch can.ko as well, the relocation never gets applied
and the call goes off into the weeds.

Name the intermediate section __klp_relocs.<objname> so post-link can
read the patched object's name from there.

Fixes: dd590d4d57eb ("objtool/klp: Introduce klp diff subcommand for diffing object files")
Reported-by: Joe Lawrence <joe.lawrence@redhat.com>
Link: https://lore.kernel.org/20260720145658.1103243-2-joe.lawrence@redhat.com
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 tools/objtool/include/objtool/klp.h | 10 ++++--
 tools/objtool/klp-diff.c            | 17 +++++++--
 tools/objtool/klp-post-link.c       | 53 +++++++++++++++++------------
 3 files changed, 52 insertions(+), 28 deletions(-)

diff --git a/tools/objtool/include/objtool/klp.h b/tools/objtool/include/objtool/klp.h
index 0118c2c170c3f..646d8e1f12eff 100644
--- a/tools/objtool/include/objtool/klp.h
+++ b/tools/objtool/include/objtool/klp.h
@@ -14,11 +14,15 @@
 #define KLP_FUNCS_SEC	".init.klp_funcs"
 
 /*
- * __klp_relocs is an intermediate section which are created by klp diff and
- * converted into KLP symbols/relas by "objtool klp post-link".  This is needed
- * to work around the linker, which doesn't preserve SHN_LIVEPATCH or
+ * __klp_relocs.<objname> are intermediate sections which are created by klp
+ * diff and converted into KLP symbols/relas by "objtool klp post-link".  This
+ * is needed to work around the linker, which doesn't preserve SHN_LIVEPATCH or
  * SHF_RELA_LIVEPATCH, nor does it support having two RELA sections for a
  * single PROGBITS section.
+ *
+ * "objname" is the name of the object being patched ("vmlinux" or a module
+ * name).  post-link uses it to name the resulting
+ * .klp.rela.objname.section_name sections.
  */
 #define KLP_RELOCS_SEC	"__klp_relocs"
 #define KLP_STRINGS_SEC	".rodata.klp.str1.1"
diff --git a/tools/objtool/klp-diff.c b/tools/objtool/klp-diff.c
index 3923fabc13331..e2c6c69dbb4f0 100644
--- a/tools/objtool/klp-diff.c
+++ b/tools/objtool/klp-diff.c
@@ -1380,8 +1380,8 @@ static int clone_reloc_klp(struct elfs *e, struct reloc *patched_reloc,
 	}
 
 	/*
-	 * Create the __klp_relocs entry.  This will be converted to an actual
-	 * KLP rela by "objtool klp post-link".
+	 * Create the __klp_relocs.<objname> entry.  This will be converted to
+	 * an actual KLP rela by "objtool klp post-link".
 	 *
 	 * This intermediate step is necessary to prevent corruption by the
 	 * linker, which doesn't know how to properly handle two rela sections
@@ -1389,7 +1389,18 @@ static int clone_reloc_klp(struct elfs *e, struct reloc *patched_reloc,
 	 */
 
 	if (!klp_relocs) {
-		klp_relocs = elf_create_section(e->out, KLP_RELOCS_SEC, 0,
+		const char *objname = find_modname(e);
+		char sec_name[SEC_NAME_LEN];
+
+		if (!objname)
+			return -1;
+
+		/* section format: __klp_relocs.objname */
+		if (snprintf_check(sec_name, SEC_NAME_LEN,
+				   KLP_RELOCS_SEC ".%s", objname))
+			return -1;
+
+		klp_relocs = elf_create_section(e->out, sec_name, 0,
 						0, SHT_PROGBITS, 8, SHF_ALLOC);
 		if (!klp_relocs)
 			return -1;
diff --git a/tools/objtool/klp-post-link.c b/tools/objtool/klp-post-link.c
index c013e39957b11..350d20495897b 100644
--- a/tools/objtool/klp-post-link.c
+++ b/tools/objtool/klp-post-link.c
@@ -19,19 +19,11 @@
 #include <objtool/util.h>
 #include <linux/livepatch_external.h>
 
-static int fix_klp_relocs(struct elf *elf)
+static int fix_klp_reloc_sec(struct elf *elf, struct section *symtab,
+			     struct section *klp_relocs)
 {
-	struct section *symtab, *klp_relocs;
-
-	klp_relocs = find_section_by_name(elf, KLP_RELOCS_SEC);
-	if (!klp_relocs)
-		return 0;
-
-	symtab = find_section_by_name(elf, ".symtab");
-	if (!symtab) {
-		ERROR("missing .symtab");
-		return -1;
-	}
+	/* section format: __klp_relocs.sec_objname */
+	const char *sec_objname = klp_relocs->name + strlen(KLP_RELOCS_SEC ".");
 
 	for (int i = 0; i < sec_size(klp_relocs) / sizeof(struct klp_reloc); i++) {
 		struct klp_reloc *klp_reloc;
@@ -39,7 +31,6 @@ static int fix_klp_relocs(struct elf *elf)
 		struct section *sec, *tmp, *klp_rsec;
 		unsigned long offset;
 		struct reloc *reloc;
-		char sym_modname[64];
 		char rsec_name[SEC_NAME_LEN];
 		u64 addend;
 		struct symbol *sym, *klp_sym;
@@ -55,7 +46,7 @@ static int fix_klp_relocs(struct elf *elf)
 		reloc = find_reloc_by_dest(elf, klp_relocs,
 					   klp_reloc_off + offsetof(struct klp_reloc, offset));
 		if (!reloc) {
-			ERROR("malformed " KLP_RELOCS_SEC " section");
+			ERROR("malformed %s section", klp_relocs->name);
 			return -1;
 		}
 
@@ -66,17 +57,13 @@ static int fix_klp_relocs(struct elf *elf)
 		reloc = find_reloc_by_dest(elf, klp_relocs,
 					   klp_reloc_off + offsetof(struct klp_reloc, sym));
 		if (!reloc) {
-			ERROR("malformed " KLP_RELOCS_SEC " section");
+			ERROR("malformed %s section", klp_relocs->name);
 			return -1;
 		}
 
 		klp_sym = reloc->sym;
 		addend = reloc_addend(reloc);
 
-		/* symbol format: .klp.sym.modname.sym_name,sympos */
-		if (sscanf(klp_sym->name + strlen(KLP_SYM_PREFIX), "%55[^.]", sym_modname) != 1)
-			ERROR("can't find modname in klp symbol '%s'", klp_sym->name);
-
 		/*
 		 * Create the KLP rela:
 		 */
@@ -84,7 +71,7 @@ static int fix_klp_relocs(struct elf *elf)
 		/* section format: .klp.rela.sec_objname.section_name */
 		if (snprintf_check(rsec_name, SEC_NAME_LEN,
 				   KLP_RELOC_SEC_PREFIX "%s.%s",
-				   sym_modname, sec->name))
+				   sec_objname, sec->name))
 			return -1;
 
 		klp_rsec = find_section_by_name(elf, rsec_name);
@@ -134,10 +121,32 @@ static int fix_klp_relocs(struct elf *elf)
 	return 0;
 }
 
+static int fix_klp_relocs(struct elf *elf)
+{
+	struct section *symtab, *sec;
+
+	symtab = find_section_by_name(elf, ".symtab");
+	if (!symtab) {
+		ERROR("missing .symtab");
+		return -1;
+	}
+
+	for_each_sec(elf, sec) {
+		if (strncmp(sec->name, KLP_RELOCS_SEC ".",
+			    strlen(KLP_RELOCS_SEC ".")))
+			continue;
+
+		if (fix_klp_reloc_sec(elf, symtab, sec))
+			return -1;
+	}
+
+	return 0;
+}
+
 /*
  * This runs on the livepatch module after all other linking has been done.  It
- * converts the intermediate __klp_relocs section into proper KLP relocs to be
- * processed by livepatch.  This needs to run last to avoid linker wreckage.
+ * converts the intermediate __klp_relocs.* sections into proper KLP relocs to
+ * be processed by livepatch.  This needs to run last to avoid linker wreckage.
  * Linkers don't tend to handle the "two rela sections for a single base
  * section" case very well, nor do they appreciate SHN_LIVEPATCH.
  */
-- 
2.54.0

[PATCH v3 6/9] objtool/klp: Don't match local symbols against exports

[Josh Poimboeuf]

While cloning a reloc, klp diff calls find_export() to determine whether
the referenced symbol is exported.  That decides whether the reference
needs a klp reloc, which object the klp symbol belongs to, and whether
the symbol's data needs to be copied into the patch module.

But find_export() matches purely on symbol name, so a static function or
variable which happens to share its name with an export is mistaken for
a reference to that export:

  - klp_reloc_needed() creates a klp reloc pointing at the exporting
    module's symbol rather than the local one.  For a vmlinux export it
    skips the klp reloc altogether, leaving a normal reloc which the
    module loader resolves to the vmlinux symbol.

  - clone_reloc() treats the symbol as external and clones it without
    its data, leaving a dangling reference.

  - validate_special_section_klp_reloc() attributes a static branch or
    call key to the wrong module, and for a vmlinux export skips the
    unsupported-key check entirely.

Exports are always global, so ignore local symbols in find_export().

Fixes: dd590d4d57eb ("objtool/klp: Introduce klp diff subcommand for diffing object files")
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 tools/objtool/klp-diff.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/objtool/klp-diff.c b/tools/objtool/klp-diff.c
index e2c6c69dbb4f0..f5d5711623f03 100644
--- a/tools/objtool/klp-diff.c
+++ b/tools/objtool/klp-diff.c
@@ -1101,6 +1101,9 @@ static struct export *find_export(struct symbol *sym)
 {
 	struct export *export;
 
+	if (is_local_sym(sym))
+		return NULL;
+
 	hash_for_each_possible(exports, export, hash, str_hash(sym->name)) {
 		if (!strcmp(export->sym, sym->name))
 			return export;
-- 
2.54.0

Re: [PATCH v3 6/9] objtool/klp: Don't match local symbols against exports

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Unexported global symbols that share a name with an export are incorrectly matched against the global exports list, causing misrouted livepatch relocations.
--

--- Patch [6]: [PATCH v3 6/9] objtool/klp: Don't match local symbols against exports ---
commit 91941e947fdde72be13c7760463244a153a175ce
Author: Josh Poimboeuf <jpoimboe@kernel.org>

objtool/klp: Don't match local symbols against exports

This commit updates find_export() in klp-diff.c to ignore local symbols.
This addresses an issue where static functions or variables that share a
name with an export are incorrectly treated as export references during
livepatch generation.

> diff --git a/tools/objtool/klp-diff.c b/tools/objtool/klp-diff.c
> --- a/tools/objtool/klp-diff.c
> +++ b/tools/objtool/klp-diff.c
> @@ -1101,6 +1101,9 @@ static struct export *find_export(struct symbol *sym)
>  {
>  	struct export *export;
>  
> +	if (is_local_sym(sym))
> +		return NULL;
> +
>  	hash_for_each_possible(exports, export, hash, str_hash(sym->name)) {
>  		if (!strcmp(export->sym, sym->name))
>  			return export;

[Severity: High]
This is a pre-existing issue, but does this still leave unexported global
symbols vulnerable to incorrect matching?

If a module contains an unexported global symbol that shares its name with
an unrelated exported symbol, it will bypass the is_local_sym() check.

When the module is livepatched and klp-diff processes a relocation to this
internal symbol, find_export() will perform a string match and return the
unrelated export. Would this cause a KLP relocation to incorrectly point to
the external module's export instead of the internal symbol?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/cover.1786138493.git.jpoimboe@kernel.org?part=6

Re: [PATCH v3 6/9] objtool/klp: Don't match local symbols against exports

[Josh Poimboeuf]

On Fri, Aug 07, 2026 at 09:46:11PM +0000, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> 
> Pre-existing issues:
> - [High] Unexported global symbols that share a name with an export are incorrectly matched against the global exports list, causing misrouted livepatch relocations.
> --
> 
> --- Patch [6]: [PATCH v3 6/9] objtool/klp: Don't match local symbols against exports ---
> commit 91941e947fdde72be13c7760463244a153a175ce
> Author: Josh Poimboeuf <jpoimboe@kernel.org>
> 
> objtool/klp: Don't match local symbols against exports
> 
> This commit updates find_export() in klp-diff.c to ignore local symbols.
> This addresses an issue where static functions or variables that share a
> name with an export are incorrectly treated as export references during
> livepatch generation.
> 
> > diff --git a/tools/objtool/klp-diff.c b/tools/objtool/klp-diff.c
> > --- a/tools/objtool/klp-diff.c
> > +++ b/tools/objtool/klp-diff.c
> > @@ -1101,6 +1101,9 @@ static struct export *find_export(struct symbol *sym)
> >  {
> >  	struct export *export;
> >  
> > +	if (is_local_sym(sym))
> > +		return NULL;
> > +
> >  	hash_for_each_possible(exports, export, hash, str_hash(sym->name)) {
> >  		if (!strcmp(export->sym, sym->name))
> >  			return export;
> 
> [Severity: High]
> This is a pre-existing issue, but does this still leave unexported global
> symbols vulnerable to incorrect matching?
> 
> If a module contains an unexported global symbol that shares its name with
> an unrelated exported symbol, it will bypass the is_local_sym() check.
> 
> When the module is livepatched and klp-diff processes a relocation to this
> internal symbol, find_export() will perform a string match and return the
> unrelated export. Would this cause a KLP relocation to incorrectly point to
> the external module's export instead of the internal symbol?

I'm not convinced this is a realistic scenario (module having a global
symbol with the same name as an exported symbol), but as mentioned it's
a preexisting issue that can be fixed in a followup.

-- 
Josh

[PATCH v3 7/9] objtool/klp: Allow new references to module exports

[Josh Poimboeuf]

From: Joe Lawrence <joe.lawrence@redhat.com>

klp_reloc_needed() returns true for module exports to support
late-module patching.  However, clone_reloc_klp() unconditionally
rejects symbols without a twin (i.e., new references added by the
patch), even when the symbol is a known export from Module.symvers.

Relax the check: allow new references to exported symbols by only
erroring on !twin when there is no export.  The export metadata from
Module.symvers provides sufficient context to emit the klp-relocation
without a twin.

For a module export that isn't sufficient on its own though, as the
resulting klp relocation will only be resolved at patch-enable time if
the exporting module is loaded.

If the original (unpatched) module already depends on the exporting
module, the dependency is safe: the module loader ensures the dependency
is satisfied before the patched module can be loaded, so the
klp relocation target will exist.

However, if the patch introduces a reference to a module that the
original doesn't depend on, there is no such guarantee.  The exporting
module could be absent or could be unloaded at any time, leading to a
relocation failure or use-after-free.

So also add a build-time check: when a new symbol reference (no twin)
targets a module export, verify that the original module already has at
least one UNDEF symbol resolving to that same exporting module.  If not,
error out with a diagnostic message.

Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 tools/objtool/klp-diff.c | 35 +++++++++++++++++++++++++++++++++--
 1 file changed, 33 insertions(+), 2 deletions(-)

diff --git a/tools/objtool/klp-diff.c b/tools/objtool/klp-diff.c
index f5d5711623f03..6d34186d8b24c 100644
--- a/tools/objtool/klp-diff.c
+++ b/tools/objtool/klp-diff.c
@@ -1294,6 +1294,28 @@ static int convert_reloc_sym(struct elf *elf, struct reloc *reloc)
 	return convert_reloc_secsym_to_sym(elf, reloc);
 }
 
+/*
+ * Check if the original module already has a dependency on dep_mod, i.e. it
+ * already references at least one export from that module.
+ */
+static bool has_module_dep(struct elfs *e, const char *dep_mod)
+{
+	struct symbol *sym;
+
+	for_each_sym(e->orig, sym) {
+		struct export *exp;
+
+		if (!is_undef_sym(sym) || is_weak_sym(sym))
+			continue;
+
+		exp = find_export(sym);
+		if (exp && !strcmp(exp->mod, dep_mod))
+			return true;
+	}
+
+	return false;
+}
+
 /*
  * Convert a regular relocation to a klp relocation (sort of).
  */
@@ -1313,8 +1335,17 @@ static int clone_reloc_klp(struct elfs *e, struct reloc *patched_reloc,
 	unsigned long sympos;
 
 	if (!patched_sym->twin) {
-		ERROR("unexpected klp reloc for new symbol %s", patched_sym->name);
-		return -1;
+		if (!export) {
+			ERROR("unexpected klp reloc for new symbol %s", patched_sym->name);
+			return -1;
+		}
+
+		if (strcmp(export->mod, "vmlinux") &&
+		    !has_module_dep(e, export->mod)) {
+			ERROR("%s: new reference to %s (exported by %s) would create an undeclared module dependency",
+			      patched_sym->name, export->sym, export->mod);
+			return -1;
+		}
 	}
 
 	/*
-- 
2.54.0

[PATCH v3 8/9] objtool/klp: Fix relocations for EXPORT_SYMBOL_FOR_MODULES() symbols

[Josh Poimboeuf]

EXPORT_SYMBOL_FOR_MODULES() puts a symbol in a "module:<names>"
namespace, which the module loader grants access to by matching the
importing module's name against that list.

klp_reloc_needed() only creates a klp reloc for module-owned exports; a
vmlinux export gets a normal reloc.  For a vmlinux symbol exported with
EXPORT_SYMBOL_FOR_MODULES(), using a normal reloc results in a modpost
failure in klp-build:

  ERROR: modpost: module livepatch-foo uses symbol mpol_shared_policy_lookup from namespace module:kvm, but does not import it.

And the modpost error is correct: even with that error removed, the
patch module would fail to load:

  livepatch_foo: module uses symbol (mpol_shared_policy_lookup) from namespace module:kvm, but does not import it.
  livepatch_foo: Unknown symbol mpol_shared_policy_lookup (err -22)

Treat it like an unexported symbol by using a klp reloc.

Note this only affects "module:" namespaces.  Ordinary namespaced
exports continue to work with normal relocs thanks to copy_import_ns(),
which propagates the patched object's import_ns tags to the patch
module.

Fixes: dd590d4d57eb ("objtool/klp: Introduce klp diff subcommand for diffing object files")
Reported-by: Joe Lawrence <joe.lawrence@redhat.com>
Link: https://lore.kernel.org/6a6608f4-0a05-4d75-8b7f-edddfac9c5d4@redhat.com
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 tools/objtool/klp-diff.c | 28 +++++++++++++++++++++++-----
 1 file changed, 23 insertions(+), 5 deletions(-)

diff --git a/tools/objtool/klp-diff.c b/tools/objtool/klp-diff.c
index 6d34186d8b24c..0f135b74a5b0c 100644
--- a/tools/objtool/klp-diff.c
+++ b/tools/objtool/klp-diff.c
@@ -30,7 +30,9 @@ struct elfs {
 
 struct export {
 	struct hlist_node hash;
-	char *mod, *sym;
+	char *mod;
+	char *sym;
+	bool mod_ns;
 };
 
 bool debug, debug_correlate, debug_clone;
@@ -135,7 +137,7 @@ static int read_exports(void)
 	}
 
 	while (fgets(line, 1024, file)) {
-		char *sym, *mod, *type;
+		char *sym, *mod, *type, *namespace;
 		struct export *export;
 
 		sym = strchr(line, '\t');
@@ -162,6 +164,14 @@ static int read_exports(void)
 
 		*type++ = '\0';
 
+		namespace = strchr(type, '\t');
+		if (!namespace) {
+			ERROR("malformed Module.symvers (namespace) at line %d", line_num);
+			return -1;
+		}
+
+		*namespace++ = '\0';
+
 		if (*sym == '\0' || *mod == '\0') {
 			ERROR("malformed Module.symvers at line %d", line_num);
 			return -1;
@@ -188,6 +198,9 @@ static int read_exports(void)
 			return -1;
 		}
 
+		/* EXPORT_SYMBOL_FOR_MODULES() */
+		export->mod_ns = strstarts(namespace, "module:");
+
 		hash_add(exports, &export->hash, str_hash(sym));
 	}
 
@@ -1174,11 +1187,16 @@ static bool klp_reloc_needed(struct reloc *patched_reloc)
 	 * clusterfunk that is late module patching, the patch module is
 	 * allowed to be loaded before any modules it depends on.
 	 *
-	 * If exported by vmlinux, a normal reloc will do.
+	 * If exported by vmlinux to all modules, a normal reloc will do.
 	 */
 	export = find_export(patched_sym);
-	if (export)
-		return strcmp(export->mod, "vmlinux");
+	if (export) {
+		if (strcmp(export->mod, "vmlinux"))
+			return true;
+
+		/* EXPORT_SYMBOL_FOR_MODULES() gets a klp reloc */
+		return export->mod_ns;
+	}
 
 	if (!patched_sym->twin) {
 		/*
-- 
2.54.0

[PATCH v3 9/9] objtool/klp: Fix line numbers in Module.symvers parse errors

[Josh Poimboeuf]

read_exports() reports the offending line number when it fails to parse
Module.symvers.  The counter is initialized to 1 but never incremented,
so every error blames line 1 regardless of where the bad line is.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 tools/objtool/klp-diff.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/klp-diff.c b/tools/objtool/klp-diff.c
index 0f135b74a5b0c..b6b72ed71bf02 100644
--- a/tools/objtool/klp-diff.c
+++ b/tools/objtool/klp-diff.c
@@ -118,7 +118,7 @@ static int read_exports(void)
 {
 	const char *symvers = "Module.symvers";
 	char line[1024], *path = NULL;
-	unsigned int line_num = 1;
+	unsigned int line_num = 0;
 	FILE *file;
 
 	file = fopen(symvers, "r");
@@ -140,6 +140,8 @@ static int read_exports(void)
 		char *sym, *mod, *type, *namespace;
 		struct export *export;
 
+		line_num++;
+
 		sym = strchr(line, '\t');
 		if (!sym) {
 			ERROR("malformed Module.symvers (sym) at line %d", line_num);
-- 
2.54.0