On Wed, May 12, 2021 at 01:19:47PM -0000, tip-bot2 for Peter Zijlstra wrote:
> The following commit has been merged into the objtool/core branch of tip:
>
> Commit-ID: ab3257042c26d0cd44793c741e2f89bf38b21fe8
> Gitweb: https://git.kernel.org/tip/ab3257042c26d0cd44793c741e2f89bf38b21fe8
> Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> AuthorDate: Thu, 06 May 2021 21:34:05 +02:00
> Committer: Ingo Molnar <mingo@xxxxxxxxxx>
> CommitterDate: Wed, 12 May 2021 14:54:56 +02:00
>
> jump_label, x86: Allow short NOPs
>
> Now that objtool is able to rewrite jump_label instructions, have the
> compiler emit a JMP, such that it can decide on the optimal encoding,
> and set jump_entry::key bit1 to indicate that objtool should rewrite
> the instruction to a matching NOP.
>
> For x86_64-allyesconfig this gives:
>
> jl\ NOP JMP
> short: 22997 124
> long: 30874 90
>
> IOW, we save (22997+124) * 3 bytes of kernel text in hotpaths.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> Link: https://lore.kernel.org/r/20210506194158.216763632@xxxxxxxxxxxxx
So Willy is having some trouble with this commit; for some reason his
kernel is no longer booting in his qemu thing, but I can't reproduce.
I've hacked up the below vmlinux.o validation, willy can you run this on
your vmlinux.o, something like:
build/tools/objtool/objtool check -abdJsuld build/vmlinux.o
Where I'm assuming you build with O=build/. When I run it on my build
(with your .config) I get absolutely nothing :/
Alternatively, can you get me your vmlinux.o + bzImage ?
Also helpful might be trying to attach gdb to the qemu gdbstub and
looking where the boot fails.
---
diff --git a/tools/objtool/builtin-check.c b/tools/objtool/builtin-check.c
index 8b38b5d6fec7..100f3efa6136 100644
--- a/tools/objtool/builtin-check.c
+++ b/tools/objtool/builtin-check.c
@@ -20,7 +20,7 @@
#include <objtool/objtool.h>
bool no_fp, no_unreachable, retpoline, module, backtrace, uaccess, stats,
- validate_dup, vmlinux, mcount, noinstr, backup;
+ validate_dup, vmlinux, mcount, noinstr, backup, validate_jl;
static const char * const check_usage[] = {
"objtool check [<options>] file.o",
@@ -45,6 +45,7 @@ const struct option check_options[] = {
OPT_BOOLEAN('l', "vmlinux", &vmlinux, "vmlinux.o validation"),
OPT_BOOLEAN('M', "mcount", &mcount, "generate __mcount_loc"),
OPT_BOOLEAN('B', "backup", &backup, "create .orig files before modification"),
+ OPT_BOOLEAN('J', "jump-label", &validate_jl, "validate jump-label tables"),
OPT_END(),
};
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 2c6a93edf27e..c3c82e40cbee 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1225,6 +1225,33 @@ static int handle_jump_alt(struct objtool_file *file,
struct instruction *orig_insn,
struct instruction **new_insn)
{
+ if (validate_jl) {
+#if 0
+ if (special_alt->key_addend & 2) {
+ WARN_FUNC("jump-label mod: %s", orig_insn->sec, orig_insn->offset,
+ orig_insn->type == INSN_NOP ? "nop" : "jmp");
+ }
+#endif
+
+ if (orig_insn->len == 2) {
+ s32 disp;
+
+ if (special_alt->orig_sec != special_alt->new_sec) {
+ WARN_FUNC("short jump-label cannot cross sections",
+ orig_insn->sec, orig_insn->offset);
+ return -1;
+ }
+
+ disp = special_alt->new_off - (special_alt->orig_off + 2);
+
+ if ((disp >> 31) != (disp >> 7)) {
+ WARN_FUNC("short jump-label, displacement too large: 0x%08x",
+ orig_insn->sec, orig_insn->offset, disp);
+ return -1;
+ }
+ }
+ }
+
if (orig_insn->type == INSN_NOP) {
do_nop:
if (orig_insn->len == 2)
@@ -1244,6 +1271,11 @@ static int handle_jump_alt(struct objtool_file *file,
if (special_alt->key_addend & 2) {
struct reloc *reloc = insn_reloc(file, orig_insn);
+ if (validate_jl) {
+ WARN_FUNC("jump-label unpatched", orig_insn->sec, orig_insn->offset);
+ return -1;
+ }
+
if (reloc) {
reloc->type = R_NONE;
elf_write_reloc(file->elf, reloc);
@@ -1341,6 +1373,8 @@ static int add_special_section_alts(struct objtool_file *file)
}
if (stats) {
+ if (validate_jl)
+ printf("validate-");
printf("jl\\\tNOP\tJMP\n");
printf("short:\t%ld\t%ld\n", file->jl_nop_short, file->jl_short);
printf("long:\t%ld\t%ld\n", file->jl_nop_long, file->jl_long);
diff --git a/tools/objtool/include/objtool/builtin.h b/tools/objtool/include/objtool/builtin.h
index 15ac0b7d3d6a..c9a00423ebd5 100644
--- a/tools/objtool/include/objtool/builtin.h
+++ b/tools/objtool/include/objtool/builtin.h
@@ -9,7 +9,7 @@
extern const struct option check_options[];
extern bool no_fp, no_unreachable, retpoline, module, backtrace, uaccess, stats,
- validate_dup, vmlinux, mcount, noinstr, backup;
+ validate_dup, vmlinux, mcount, noinstr, backup, validate_jl;
extern int cmd_parse_options(int argc, const char **argv, const char * const usage[]);
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