4.16-stable review patch. If anyone has any objections, please let me know.
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From: Josh Poimboeuf jpoimboe@redhat.com
commit 7dec80ccbe310fb7e225bf21c48c672bb780ce7b upstream.
With the following commit:
fd35c88b7417 ("objtool: Support GCC 8 switch tables")
I added a "can't find switch jump table" warning, to stop covering up silent failures if add_switch_table() can't find anything.
That warning found yet another bug in the objtool switch table detection logic. For cases 1 and 2 (as described in the comments of find_switch_table()), the find_symbol_containing() check doesn't adjust the offset for RIP-relative switch jumps.
Incidentally, this bug was already fixed for case 3 with:
6f5ec2993b1f ("objtool: Detect RIP-relative switch table references")
However, that commit missed the fix for cases 1 and 2.
The different cases are now starting to look more and more alike. So fix the bug by consolidating them into a single case, by checking the original dynamic jump instruction in the case 3 loop.
This also simplifies the code and makes it more robust against future switch table detection issues -- of which I'm sure there will be many...
Switch table detection has been the most fragile area of objtool, by far. I long for the day when we'll have a GCC plugin for annotating switch tables. Linus asked me to delay such a plugin due to the flakiness of the plugin infrastructure in older versions of GCC, so this rickety code is what we're stuck with for now. At least the code is now a little simpler than it was.
Reported-by: kbuild test robot lkp@intel.com Signed-off-by: Josh Poimboeuf jpoimboe@redhat.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Peter Zijlstra peterz@infradead.org Cc: Thomas Gleixner tglx@linutronix.de Link: http://lkml.kernel.org/r/f400541613d45689086329432f3095119ffbc328.1526674218... Signed-off-by: Ingo Molnar mingo@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- tools/objtool/check.c | 37 ++++++++++++------------------------- 1 file changed, 12 insertions(+), 25 deletions(-)
--- a/tools/objtool/check.c +++ b/tools/objtool/check.c @@ -901,40 +901,19 @@ static struct rela *find_switch_table(st struct instruction *orig_insn = insn; unsigned long table_offset;
- /* case 1 & 2 */ - text_rela = find_rela_by_dest_range(insn->sec, insn->offset, insn->len); - if (text_rela && text_rela->sym == file->rodata->sym && - !find_symbol_containing(file->rodata, text_rela->addend)) { - - table_offset = text_rela->addend; - if (text_rela->type == R_X86_64_PC32) { - /* case 2 */ - table_offset += 4; - file->ignore_unreachables = true; - } - - rodata_rela = find_rela_by_dest(file->rodata, table_offset); - if (!rodata_rela) - return NULL; - - return rodata_rela; - } - - /* case 3 */ /* * Backward search using the @first_jump_src links, these help avoid * much of the 'in between' code. Which avoids us getting confused by * it. */ - for (insn = list_prev_entry(insn, list); - + for (; &insn->list != &file->insn_list && insn->sec == func->sec && insn->offset >= func->offset;
insn = insn->first_jump_src ?: list_prev_entry(insn, list)) {
- if (insn->type == INSN_JUMP_DYNAMIC) + if (insn != orig_insn && insn->type == INSN_JUMP_DYNAMIC) break;
/* allow small jumps within the range */ @@ -961,10 +940,18 @@ static struct rela *find_switch_table(st if (find_symbol_containing(file->rodata, table_offset)) continue;
- /* mov [rodata addr], %reg */ rodata_rela = find_rela_by_dest(file->rodata, table_offset); - if (rodata_rela) + if (rodata_rela) { + /* + * Use of RIP-relative switch jumps is quite rare, and + * indicates a rare GCC quirk/bug which can leave dead + * code behind. + */ + if (text_rela->type == R_X86_64_PC32) + file->ignore_unreachables = true; + return rodata_rela; + } }
return NULL;