2022-03-11 22:01:21

by Alexey Dobriyan

Subject: [PATCH 4/5] x86/alternative: make .altinstr_replacement section non-executable

Instructions in .altinstr_replacement section aren't meant to be
executed by CPU directly, only after being copied to the real
instruction stream in .text/.init_text.

Signed-off-by: Alexey Dobriyan (CloudLinux) <[email protected]>
---
arch/x86/include/asm/alternative.h | 10 +++++-----
tools/objtool/check.c | 2 +-
2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index cf7722a106b3..d754a18edc99 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -165,7 +165,7 @@ static inline int alternatives_text_reserved(void *start, void *end)
".pushsection .altinstructions,\"aM\",@progbits," __stringify(sizeof_struct_alt_instr) "\n"\
ALTINSTR_ENTRY(feature, 1) \
".popsection\n" \
- ".pushsection .altinstr_replacement, \"ax\"\n" \
+ ".pushsection .altinstr_replacement, \"a\"\n" \
ALTINSTR_REPLACEMENT(newinstr, 1) \
".popsection\n"

@@ -175,7 +175,7 @@ static inline int alternatives_text_reserved(void *start, void *end)
ALTINSTR_ENTRY(feature1, 1) \
ALTINSTR_ENTRY(feature2, 2) \
".popsection\n" \
- ".pushsection .altinstr_replacement, \"ax\"\n" \
+ ".pushsection .altinstr_replacement, \"a\"\n" \
ALTINSTR_REPLACEMENT(newinstr1, 1) \
ALTINSTR_REPLACEMENT(newinstr2, 2) \
".popsection\n"
@@ -192,7 +192,7 @@ static inline int alternatives_text_reserved(void *start, void *end)
ALTINSTR_ENTRY(feat2, 2) \
ALTINSTR_ENTRY(feat3, 3) \
".popsection\n" \
- ".pushsection .altinstr_replacement, \"ax\"\n" \
+ ".pushsection .altinstr_replacement, \"a\"\n" \
ALTINSTR_REPLACEMENT(newinsn1, 1) \
ALTINSTR_REPLACEMENT(newinsn2, 2) \
ALTINSTR_REPLACEMENT(newinsn3, 3) \
@@ -338,7 +338,7 @@ static inline int alternatives_text_reserved(void *start, void *end)
altinstruction_entry 140b,143f,\feature,142b-140b,144f-143f
.popsection

- .pushsection .altinstr_replacement,"ax"
+ .pushsection .altinstr_replacement,"a"
143:
\newinstr
144:
@@ -376,7 +376,7 @@ static inline int alternatives_text_reserved(void *start, void *end)
altinstruction_entry 140b,144f,\feature2,142b-140b,145f-144f
.popsection

- .pushsection .altinstr_replacement,"ax"
+ .pushsection .altinstr_replacement,"a"
143:
\newinstr1
144:
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 7c33ec67c4a9..18c32035f41c 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -359,7 +359,7 @@ static int decode_instructions(struct objtool_file *file)

for_each_sec(file, sec) {

- if (!(sec->sh.sh_flags & SHF_EXECINSTR))
+ if (!(sec->sh.sh_flags & SHF_EXECINSTR) && strcmp(sec->name, ".altinstr_replacement"))
continue;

if (strcmp(sec->name, ".altinstr_replacement") &&
--
2.34.1

2022-03-12 20:14:54

by Peter Zijlstra

Subject: Re: [PATCH 4/5] x86/alternative: make .altinstr_replacement section non-executable

On Fri, Mar 11, 2022 at 05:43:11PM +0300, Alexey Dobriyan wrote:
> Instructions in .altinstr_replacement section aren't meant to be
> executed by CPU directly, only after being copied to the real
> instruction stream in .text/.init_text.

But who cares; the entire section is deleted right along with
.init_text.