Re: [PATCH v2 42/62] kbuild,x86: Fix module permissions for __jump_table and __bug_table
On Wed, Jun 04, 2025 at 05:22:15PM -0700, Josh Poimboeuf wrote: > On Mon, May 26, 2025 at 01:06:34PM +0200, Peter Zijlstra wrote: > > On Fri, May 09, 2025 at 01:17:06PM -0700, Josh Poimboeuf wrote: > > > An upcoming patch will add the SHF_MERGE flag to x86 __jump_table and > > > __bug_table so their entry sizes can be defined in inline asm. > > > > > > However, those sections have SHF_WRITE, which the Clang linker (lld) > > > explicitly forbids combining with SHF_MERGE. > > > > > > Those sections are modified at runtime and must remain writable. While > > > SHF_WRITE is ignored by vmlinux, it's still needed for modules. > > > > > > To work around the linker interference, remove SHF_WRITE during > > > compilation and restore it after linking the module. > > > > *groan* > > > > This and the following patches marking a whole bunch of sections M, > > seems to suggest you're going to rely on sh_entsize actually working. > > > > There was an ld.lld bug, and IIRC you need to enforce llvm-20 or later > > if you want this to be so. > > Hm, ISTR this working with clang 18, I'll go test that again. You're right, looks like sh_entsize is getting cleared by the linker with my Clang 18. I guess I tested with newer Clang. "objtool klp diff" fails with: vmlinux.o: error: objtool: .discard.annotate_insn: unknown entry size So yeah, non-buggy linker is already being enforced, though I should probably make the error more human friendly. -- Josh
Re: [PATCH v2 42/62] kbuild,x86: Fix module permissions for __jump_table and __bug_table
On Mon, May 26, 2025 at 01:06:34PM +0200, Peter Zijlstra wrote: > On Fri, May 09, 2025 at 01:17:06PM -0700, Josh Poimboeuf wrote: > > An upcoming patch will add the SHF_MERGE flag to x86 __jump_table and > > __bug_table so their entry sizes can be defined in inline asm. > > > > However, those sections have SHF_WRITE, which the Clang linker (lld) > > explicitly forbids combining with SHF_MERGE. > > > > Those sections are modified at runtime and must remain writable. While > > SHF_WRITE is ignored by vmlinux, it's still needed for modules. > > > > To work around the linker interference, remove SHF_WRITE during > > compilation and restore it after linking the module. > > *groan* > > This and the following patches marking a whole bunch of sections M, > seems to suggest you're going to rely on sh_entsize actually working. > > There was an ld.lld bug, and IIRC you need to enforce llvm-20 or later > if you want this to be so. Hm, ISTR this working with clang 18, I'll go test that again. -- Josh
Re: [PATCH v2 42/62] kbuild,x86: Fix module permissions for __jump_table and __bug_table
On Fri, May 09, 2025 at 01:17:06PM -0700, Josh Poimboeuf wrote: > An upcoming patch will add the SHF_MERGE flag to x86 __jump_table and > __bug_table so their entry sizes can be defined in inline asm. > > However, those sections have SHF_WRITE, which the Clang linker (lld) > explicitly forbids combining with SHF_MERGE. > > Those sections are modified at runtime and must remain writable. While > SHF_WRITE is ignored by vmlinux, it's still needed for modules. > > To work around the linker interference, remove SHF_WRITE during > compilation and restore it after linking the module. *groan* This and the following patches marking a whole bunch of sections M, seems to suggest you're going to rely on sh_entsize actually working. There was an ld.lld bug, and IIRC you need to enforce llvm-20 or later if you want this to be so.
[PATCH v2 42/62] kbuild,x86: Fix module permissions for __jump_table and __bug_table
An upcoming patch will add the SHF_MERGE flag to x86 __jump_table and
__bug_table so their entry sizes can be defined in inline asm.
However, those sections have SHF_WRITE, which the Clang linker (lld)
explicitly forbids combining with SHF_MERGE.
Those sections are modified at runtime and must remain writable. While
SHF_WRITE is ignored by vmlinux, it's still needed for modules.
To work around the linker interference, remove SHF_WRITE during
compilation and restore it after linking the module.
Cc: Masahiro Yamada
Signed-off-by: Josh Poimboeuf
---
arch/Kconfig | 3 +++
arch/x86/Kconfig | 1 +
arch/x86/include/asm/bug.h| 4 ++--
arch/x86/include/asm/jump_label.h | 2 +-
scripts/Makefile.modfinal | 18 +-
5 files changed, 20 insertions(+), 8 deletions(-)
diff --git a/arch/Kconfig b/arch/Kconfig
index b0adb665041f..a413cd86f87c 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1314,6 +1314,9 @@ config HAVE_NOINSTR_HACK
config HAVE_NOINSTR_VALIDATION
bool
+config NEED_MODULE_PERMISSIONS_FIX
+ bool
+
config HAVE_UACCESS_VALIDATION
bool
select OBJTOOL
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 4c33c644b92d..996d59e59e5d 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -309,6 +309,7 @@ config X86
select HOTPLUG_SPLIT_STARTUPif SMP && X86_32
select IRQ_FORCED_THREADING
select LOCK_MM_AND_FIND_VMA
+ select NEED_MODULE_PERMISSIONS_FIX
select NEED_PER_CPU_EMBED_FIRST_CHUNK
select NEED_PER_CPU_PAGE_FIRST_CHUNK
select NEED_SG_DMA_LENGTH
diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
index f0e9acf72547..fb3534ddbea2 100644
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -42,7 +42,7 @@
#define _BUG_FLAGS(ins, flags, extra) \
do { \
asm_inline volatile("1:\t" ins "\n" \
-".pushsection __bug_table,\"aw\"\n"\
+".pushsection __bug_table,\"a\"\n" \
"2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n" \
"\t" __BUG_REL(%c0) "\t# bug_entry::file\n" \
"\t.word %c1""\t# bug_entry::line\n" \
@@ -60,7 +60,7 @@ do {
\
#define _BUG_FLAGS(ins, flags, extra) \
do { \
asm_inline volatile("1:\t" ins "\n" \
-".pushsection __bug_table,\"aw\"\n"\
+".pushsection __bug_table,\"a\"\n" \
"2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n" \
"\t.word %c0""\t# bug_entry::flags\n" \
"\t.org 2b+%c1\n" \
diff --git a/arch/x86/include/asm/jump_label.h
b/arch/x86/include/asm/jump_label.h
index 61dd1dee7812..cd21554b3675 100644
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -13,7 +13,7 @@
#include
#define JUMP_TABLE_ENTRY(key, label) \
- ".pushsection __jump_table, \"aw\" \n\t" \
+ ".pushsection __jump_table, \"a\"\n\t" \
_ASM_ALIGN "\n\t" \
".long 1b - . \n\t" \
".long " label " - . \n\t" \
diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index 542ba462ed3e..878d0d25a461 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal
@@ -28,12 +28,23 @@ ccflags-remove-y := $(CC_FLAGS_CFI)
.module-common.o: $(srctree)/scripts/module-common.c FORCE
$(call if_changed_rule,cc_o_c)
+ifdef CONFIG_NEED_MODULE_PERMISSIONS_FIX
+cmd_fix_mod_permissions = \
+ $(OBJCOPY) --set-section-flags __jump_table=alloc,data \
+ --set-section-flags __bug_table=alloc,data $@
+endif
+
quiet_cmd_ld_ko_o = LD [M] $@
cmd_ld_ko_o =\
$(LD) -r $(KBUILD_LDFLAGS) \
$(KBUILD_LDFLAGS_MODULE) $(LDFLAGS_MODULE) \
-T $(objtree)/scripts/module.lds -o $@ $(filter %.o, $^)
+define rule_ld_ko_o
+ $(call cmd_and_savecmd,ld_ko_o)
+ $(call cmd,fix_mod_permissions)
+endef
+
quiet_cmd_btf_ko = BTF [M] $@
cmd_btf_ko = \
if [ ! -f $(objtree)/vmlinux ]; then\
@@ -46,14 +57,11 @@ quiet_cmd_btf_ko = BTF [M] $@
# Same as newer-prereqs, but allows to
