[PATCH 0/2] arm64: Implement support for BTI veneers

[PATCH 0/2] arm64: Implement support for BTI veneers

[Ard Biesheuvel]

Recent toolchains will omit BTI landing pads from functions with static
linkage that never have their address taken. If a landing pad is needed
nonetheless, it is up to the static linker to emit a BTI veneer withing
direct branching range of the target, and direct the call to the veneer
instead.

Modules are partially linked objects, and so there is no static linker
that can do this for us. Instead, the module loader must see to this.

So implement this for the arm64 module loader. Patch #2 contains a test
module that was used to validate the approach.

Cc: Mark Brown <broonie@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>

Link: https://lore.kernel.org/all/ed4fe1f95071897859ec7fbe9176246cbd4962bf.1786138806.git.jpoimboe@kernel.org/

Ard Biesheuvel (2):
  arm64: module: Emit BTI veneers for cross-section calls
  DONOTMERGE: arm64: module: Test module for BTI veneers

 arch/arm64/Kconfig                  |   2 -
 arch/arm64/include/asm/module.h     |  12 ++
 arch/arm64/include/asm/module.lds.h |   3 +
 arch/arm64/kernel/Makefile          |   1 +
 arch/arm64/kernel/bti_veneer_test.c |  26 ++++
 arch/arm64/kernel/module-plts.c     | 128 +++++++++++++++++++-
 6 files changed, 165 insertions(+), 7 deletions(-)
 create mode 100644 arch/arm64/kernel/bti_veneer_test.c

-- 
2.47.3

[PATCH 1/2] arm64: module: Emit BTI veneers for cross-section calls

[Ard Biesheuvel]

The compiler is permitted to omit BTI landing pads from static functions
that never have their address taken, but are only called directly, even
if those calls originate from other code sections.

This means that calls into a module's .text section from .init.text,
which may need to be routed via a PLT if .text is out of direct
branching range, may result in BTI exceptions due to the indirect calls
performed by the PLT veneers. (Note that calls to .init.text from .text
are not allowed.)

The 'solution' is to emit yet another veneer - this is what the ELF
psABI for AArch64 mandates in this case.

So derive an upper bound for the number of veneers that may be needed in
the core module region to ensure that any call from init code that ends
up needing a PLT can be directed at a veneer with a BTI landing pad, and
allocate the additional space.

Then, emit these veneers as needed, i.e., only when emitting a PLT entry
for a call from an init code section to a normal code section in the
same module. In practice, this only occurs when a module's .init.text
happens to be allocated far away from its .text section, which might
happen when the initial 128M 'near' module region runs out of space
between allocating the core module and allocating its init region.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/arm64/Kconfig                  |   2 -
 arch/arm64/include/asm/module.h     |  12 ++
 arch/arm64/include/asm/module.lds.h |   3 +
 arch/arm64/kernel/module-plts.c     | 128 +++++++++++++++++++-
 4 files changed, 138 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index b3afe0688919..25fa80b5591d 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -2114,8 +2114,6 @@ config ARM64_BTI_KERNEL
 	depends on CC_HAS_BRANCH_PROT_PAC_RET_BTI
 	# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94697
 	depends on !CC_IS_GCC || GCC_VERSION >= 100100
-	# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106671
-	depends on !CC_IS_GCC
 	depends on (!FUNCTION_GRAPH_TRACER || DYNAMIC_FTRACE_WITH_ARGS)
 	help
 	  Build the kernel with Branch Target Identification annotations
diff --git a/arch/arm64/include/asm/module.h b/arch/arm64/include/asm/module.h
index fb9b88eebeb1..0bdac3db5719 100644
--- a/arch/arm64/include/asm/module.h
+++ b/arch/arm64/include/asm/module.h
@@ -16,6 +16,7 @@ struct mod_plt_sec {
 struct mod_arch_specific {
 	struct mod_plt_sec	core;
 	struct mod_plt_sec	init;
+	struct mod_plt_sec	bti;
 
 	/* for CONFIG_DYNAMIC_FTRACE */
 	struct plt_entry	*ftrace_trampolines;
@@ -43,6 +44,17 @@ struct plt_entry {
 	__le32	br;	/* br	x16				*/
 };
 
+struct bti_veneer {
+	/*
+	 * Functions with static linkage may lack BTI landing pads if their
+	 * address is never taken. E.g., a direct call from .init.text to a
+	 * static function in .text may need an additional veneer at the target
+	 * end if it is routed via a PLT entry.
+	 */
+	__le32	bti_c;
+	__le32	b;
+};
+
 static inline bool is_forbidden_offset_for_adrp(void *place)
 {
 	return cpus_have_final_cap(ARM64_WORKAROUND_843419) &&
diff --git a/arch/arm64/include/asm/module.lds.h b/arch/arm64/include/asm/module.lds.h
index 0b3aacd22c59..b6cc9da9dd24 100644
--- a/arch/arm64/include/asm/module.lds.h
+++ b/arch/arm64/include/asm/module.lds.h
@@ -1,6 +1,9 @@
 SECTIONS {
 	.plt 0 : { BYTE(0) }
 	.init.plt 0 : { BYTE(0) }
+#ifdef CONFIG_ARM64_BTI_KERNEL
+	.text.bti_veneer 0 : { BYTE(0) }
+#endif
 	.text.ftrace_trampoline 0 : { BYTE(0) }
 	.init.text.ftrace_trampoline 0 : { BYTE(0) }
 
diff --git a/arch/arm64/kernel/module-plts.c b/arch/arm64/kernel/module-plts.c
index 7afd370da9f4..e2c009cce96c 100644
--- a/arch/arm64/kernel/module-plts.c
+++ b/arch/arm64/kernel/module-plts.c
@@ -66,12 +66,48 @@ static bool plt_entries_equal(const struct plt_entry *a,
 	       (q + aarch64_insn_adrp_get_offset(le32_to_cpu(b->adrp)));
 }
 
+static u64 module_emit_bti_veneer(struct module *mod, const Elf64_Shdr *sechdrs,
+				  u64 target)
+{
+	struct mod_plt_sec *pltsec = &mod->arch.bti;
+	struct bti_veneer *btiv = (void *)sechdrs[pltsec->plt_shndx].sh_addr;
+
+	/* Look for an existing entry pointing to 'target' */
+	for (int i = 0; i < pltsec->plt_num_entries; i++) {
+		u64 dst = (u64)&btiv[i].b +
+			  aarch64_get_branch_offset(btiv[i].b);
+		if (dst == target)
+			return (u64)&btiv[i];
+	}
+
+	/* Allocate a new veneer */
+	if (WARN_ON(pltsec->plt_num_entries >= pltsec->plt_max_entries))
+		return 0;
+
+	btiv += pltsec->plt_num_entries++;
+
+	btiv->bti_c	= aarch64_insn_gen_hint(AARCH64_INSN_HINT_BTIC);
+	btiv->b		= aarch64_insn_gen_branch_imm((u64)&btiv->b, target,
+						      AARCH64_INSN_BRANCH_NOLINK);
+
+	return (u64)btiv;
+}
+
+static bool target_needs_bti_veneer(struct module *mod, const Elf64_Sym *sym)
+{
+	if (sym->st_shndx == STN_UNDEF ||
+	    ELF64_ST_BIND(sym->st_info) != STB_LOCAL)
+		return false;
+
+	return !within_module_init((unsigned long)sym->st_value, mod);
+}
+
 u64 module_emit_plt_entry(struct module *mod, Elf64_Shdr *sechdrs,
 			  void *loc, const Elf64_Rela *rela,
 			  Elf64_Sym *sym)
 {
-	struct mod_plt_sec *pltsec = !within_module_init((unsigned long)loc, mod) ?
-						&mod->arch.core : &mod->arch.init;
+	bool is_init = within_module_init((unsigned long)loc, mod);
+	struct mod_plt_sec *pltsec = !is_init ? &mod->arch.core : &mod->arch.init;
 	struct plt_entry *plt = (struct plt_entry *)sechdrs[pltsec->plt_shndx].sh_addr;
 	int i = pltsec->plt_num_entries;
 	int j = i - 1;
@@ -80,6 +116,16 @@ u64 module_emit_plt_entry(struct module *mod, Elf64_Shdr *sechdrs,
 	if (is_forbidden_offset_for_adrp(&plt[i].adrp))
 		i++;
 
+	if (system_supports_bti_kernel() && is_init) {
+		/*
+		 * Check if the target is a function with static linkage within
+		 * the same module but in a non-init section: if so, point the
+		 * PLT entry at a BTI veneer instead.
+		 */
+		if (target_needs_bti_veneer(mod, sym))
+			val = module_emit_bti_veneer(mod, sechdrs, val);
+	}
+
 	plt[i] = get_plt_entry(val, &plt[i]);
 
 	/*
@@ -277,11 +323,66 @@ static int partition_branch_plt_relas(Elf64_Sym *syms, Elf64_Rela *rela,
 	return i;
 }
 
+static int count_bti_veneers(const Elf_Ehdr *ehdr, const Elf_Shdr *sechdrs,
+			     const char *secstrings, const Elf64_Sym *syms,
+			     Elf64_Word cur)
+{
+	int count = 0;
+
+	if (!system_supports_bti_kernel())
+		return 0;
+
+	/*
+	 * BTI veneers must be emitted within direct branching range of the
+	 * target function, so that PLTs emitted to perform calls that exceed
+	 * that range can use indirect calls as usual, even if the target
+	 * function lacks a landing pad. This is needed between init code
+	 * sections and normal code sections, which can be loaded far away from
+	 * each other. It should never be needed the other way around, given
+	 * that normal code cannot call init code.
+	 *
+	 * So go over init code sections, and find call/jump relocations
+	 * referring to symbols in the current section. If the symbol has
+	 * static linkage, allocate space for a BTI veneer.
+	 */
+
+	for (int i = 0; i < ehdr->e_shnum; i++) {
+		if (sechdrs[i].sh_type != SHT_RELA)
+			continue;
+
+		const Elf64_Shdr *dstsec = sechdrs + sechdrs[i].sh_info;
+
+		/* Ignore relocations that operate on non-exec sections */
+		if (!(dstsec->sh_flags & SHF_EXECINSTR))
+			continue;
+
+		/* Only look at .init code sections */
+		if (!module_init_layout_section(secstrings + dstsec->sh_name))
+			continue;
+
+		Elf64_Rela *rela = (void *)ehdr + sechdrs[i].sh_offset;
+		int num = sechdrs[i].sh_size / sizeof(Elf64_Rela);
+		for (int j = 0; j < num; j++) {
+			const Elf64_Sym *s = syms + ELF64_R_SYM(rela[j].r_info);
+
+			switch (ELF64_R_TYPE(rela[j].r_info)) {
+			case R_AARCH64_JUMP26:
+			case R_AARCH64_CALL26:
+				if (s->st_shndx == cur &&
+				    ELF64_ST_BIND(s->st_info) == STB_LOCAL)
+					count++;
+			}
+		}
+	}
+	return count;
+}
+
 int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
 			      char *secstrings, struct module *mod)
 {
 	unsigned long core_plts = 0;
 	unsigned long init_plts = 0;
+	unsigned long bti_veneers = 0;
 	Elf64_Sym *syms = NULL;
 	Elf_Shdr *pltsec, *tramp = NULL, *init_tramp = NULL;
 	int i;
@@ -295,6 +396,8 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
 			mod->arch.core.plt_shndx = i;
 		else if (!strcmp(secstrings + sechdrs[i].sh_name, ".init.plt"))
 			mod->arch.init.plt_shndx = i;
+		else if (!strcmp(secstrings + sechdrs[i].sh_name, ".text.bti_veneer"))
+			mod->arch.bti.plt_shndx = i;
 		else if (!strcmp(secstrings + sechdrs[i].sh_name,
 				 ".text.ftrace_trampoline"))
 			tramp = sechdrs + i;
@@ -305,7 +408,8 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
 			syms = (Elf64_Sym *)sechdrs[i].sh_addr;
 	}
 
-	if (!mod->arch.core.plt_shndx || !mod->arch.init.plt_shndx) {
+	if (!mod->arch.core.plt_shndx || !mod->arch.init.plt_shndx ||
+	    (IS_ENABLED(CONFIG_ARM64_BTI_KERNEL) && !mod->arch.bti.plt_shndx)) {
 		pr_err("%s: module PLT section(s) missing\n", mod->name);
 		return -ENOEXEC;
 	}
@@ -335,12 +439,16 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
 		if (nents)
 			sort(rels, nents, sizeof(Elf64_Rela), cmp_rela, NULL);
 
-		if (!module_init_layout_section(secstrings + dstsec->sh_name))
+		if (!module_init_layout_section(secstrings + dstsec->sh_name)) {
 			core_plts += count_plts(syms, rels, numrels,
 						sechdrs[i].sh_info, dstsec);
-		else
+			bti_veneers += count_bti_veneers(ehdr, sechdrs,
+							 secstrings, syms,
+							 sechdrs[i].sh_info);
+		} else {
 			init_plts += count_plts(syms, rels, numrels,
 						sechdrs[i].sh_info, dstsec);
+		}
 	}
 
 	pltsec = sechdrs + mod->arch.core.plt_shndx;
@@ -359,6 +467,16 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
 	mod->arch.init.plt_num_entries = 0;
 	mod->arch.init.plt_max_entries = init_plts;
 
+	if (system_supports_bti_kernel()) {
+		pltsec = sechdrs + mod->arch.bti.plt_shndx;
+		pltsec->sh_type = SHT_NOBITS;
+		pltsec->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
+		pltsec->sh_addralign = L1_CACHE_BYTES;
+		pltsec->sh_size = (bti_veneers + 1) * sizeof(struct bti_veneer);
+		mod->arch.bti.plt_num_entries = 0;
+		mod->arch.bti.plt_max_entries = bti_veneers;
+	}
+
 	if (tramp) {
 		tramp->sh_type = SHT_NOBITS;
 		tramp->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
-- 
2.47.3

Re: [PATCH 1/2] arm64: module: Emit BTI veneers for cross-section calls

[Josh Poimboeuf]

On Wed, Aug 12, 2026 at 06:21:00PM +0200, Ard Biesheuvel wrote:
> The compiler is permitted to omit BTI landing pads from static functions
> that never have their address taken, but are only called directly, even
> if those calls originate from other code sections.
> 
> This means that calls into a module's .text section from .init.text,
> which may need to be routed via a PLT if .text is out of direct
> branching range, may result in BTI exceptions due to the indirect calls
> performed by the PLT veneers. (Note that calls to .init.text from .text
> are not allowed.)
> 
> The 'solution' is to emit yet another veneer - this is what the ELF
> psABI for AArch64 mandates in this case.
> 
> So derive an upper bound for the number of veneers that may be needed in
> the core module region to ensure that any call from init code that ends
> up needing a PLT can be directed at a veneer with a BTI landing pad, and
> allocate the additional space.
> 
> Then, emit these veneers as needed, i.e., only when emitting a PLT entry
> for a call from an init code section to a normal code section in the
> same module. In practice, this only occurs when a module's .init.text
> happens to be allocated far away from its .text section, which might
> happen when the initial 128M 'near' module region runs out of space
> between allocating the core module and allocating its init region.
> 
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  arch/arm64/Kconfig                  |   2 -
>  arch/arm64/include/asm/module.h     |  12 ++
>  arch/arm64/include/asm/module.lds.h |   3 +
>  arch/arm64/kernel/module-plts.c     | 128 +++++++++++++++++++-
>  4 files changed, 138 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index b3afe0688919..25fa80b5591d 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -2114,8 +2114,6 @@ config ARM64_BTI_KERNEL
>  	depends on CC_HAS_BRANCH_PROT_PAC_RET_BTI
>  	# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94697
>  	depends on !CC_IS_GCC || GCC_VERSION >= 100100
> -	# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106671
> -	depends on !CC_IS_GCC

This doesn't work for livepatch though, and removing the "depends on
!CC_IS_GCC" is a livepatch regression as it broadly increases the
likelihood of ARM64_BTI_KERNEL (default y) getting enabled.

So "livepatch broken on arm64 clang 21+" now becomes "livepatch broken on arm64".

So either ARM64_BTI_KERNEL needs to depend on !LIVEPATCH (which will
keep BTI disabled for most distros), or we need a different approach
which uses __vmalloc_node_range() to allocate a veneer within 128MB of
the target.  I can try to write that up.

-- 
Josh

[PATCH 2/2] DONOTMERGE: arm64: module: Test module for BTI veneers

[Ard Biesheuvel]

This module has a tiny .text section with a static function, which is
called by the module init hook, which is placed in a ~128M init code
section, which is therefore guaranteed to be place out of direct
branching range when CONFIG_RANDOMIZE_MODULE_REGION_FULL is disabled.

When CONFIG_ARM64_BTI_KERNEL is enabled, this forces the use of a BTI
veneer, as recent GCCs and Clangs will elide BTI landing pads for static
functions that never have their address taken, even in the presence of
cross-section calls to those functions.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 arch/arm64/kernel/Makefile          |  1 +
 arch/arm64/kernel/bti_veneer_test.c | 26 ++++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index d2690c3ec528..b74e2dbd2d5f 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -72,6 +72,7 @@ obj-$(CONFIG_ARM64_MPAM)		+= mpam.o
 obj-$(CONFIG_ARM64_MTE)			+= mte.o
 obj-y					+= vdso-wrap.o
 obj-$(CONFIG_COMPAT_VDSO)		+= vdso32-wrap.o
+obj-m					+= bti_veneer_test.o
 
 # Force dependency (vdso*-wrap.S includes vdso.so through incbin)
 $(obj)/vdso-wrap.o: $(obj)/vdso/vdso.so
diff --git a/arch/arm64/kernel/bti_veneer_test.c b/arch/arm64/kernel/bti_veneer_test.c
new file mode 100644
index 000000000000..62c20e960d8d
--- /dev/null
+++ b/arch/arm64/kernel/bti_veneer_test.c
@@ -0,0 +1,26 @@
+
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/types.h>
+
+static int noinline static_noninit_function(void)
+{
+	static volatile int ret;
+	return ret;
+}
+
+static int __init bti_veneer_test_init(void)
+{
+	return static_noninit_function();
+}
+module_init(bti_veneer_test_init);
+
+static void bti_veneer_test_exit(void)
+{
+}
+module_exit(bti_veneer_test_exit);
+
+/* Make the init code region too big to fit in 128M 'near' module region */
+asm(".section .init.padding, \"ax\", %progbits; .space (128 << 20) - 64; .previous");
+
+MODULE_LICENSE("GPL");
-- 
2.47.3