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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