Messages in this thread Patch in this message | | | Date | Mon, 3 May 2021 12:28:14 -0500 | From | Josh Poimboeuf <> | Subject | Re: [PATCH] objtool: include symbol value in check for contiguous objects |
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On Wed, Apr 28, 2021 at 11:04:08PM +0200, Vanessa Hack wrote: > While trying to adopt objtool's ability to generate ORC data for the use > in our research project, we came across a problem with the detection of > function pointers of contiguous objects introduced by commit fd35c88b7417 > ("objtool: Support GCC 8 switch tables"). Only the section and the > addend/offset of the relocation/function are compared without the actual > value of the involved symbols - false positives might be reported if the > referenced symbols are different, but are in the same section. By adding > (the value of) the referenced symbol as part of the comparison, those > false positives are no longer reported. > > Co-developed-by: Jonas Rabenstein <rabenstein@cs.fau.de> > Signed-off-by: Jonas Rabenstein <rabenstein@cs.fau.de> > Signed-off-by: Vanessa Hack <vanessa.hack@fau.de> > --- > tools/objtool/check.c | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/tools/objtool/check.c b/tools/objtool/check.c > index 1f4154f9b04b..4456f3278bb8 100644 > --- a/tools/objtool/check.c > +++ b/tools/objtool/check.c > @@ -1320,6 +1320,8 @@ static int add_jump_table(struct objtool_file *file, struct instruction *insn, > * instruction. > */ > list_for_each_entry_from(reloc, &table->sec->reloc_list, list) { > + unsigned int pfunc_offset; > + unsigned int reloc_offset; > > /* Check for the end of the table: */ > if (reloc != table && reloc->jump_table_start) > @@ -1330,8 +1332,10 @@ static int add_jump_table(struct objtool_file *file, struct instruction *insn, > break; > > /* Detect function pointers from contiguous objects: */ > + pfunc_offset = pfunc->sym.st_value + pfunc->offset; > + reloc_offset = reloc->sym->sym.st_value + reloc->addend; > if (reloc->sym->sec == pfunc->sec && > - reloc->addend == pfunc->offset) > + pfunc_offset == reloc_offset) > break;
Hi Vanessa & Jonas,
Thanks for the patch!
I'm a little confused about the issue -- do you have an example listing of .rodata relocations (e.g. from 'readelf -aW') which confused objtool?
I believe add_jump_table() -- in addition to all the other jump table code -- assumes that reloc->sym is a section symbol (STT_SECTION), rather than a function symbol (STT_FUNC). Was it an STT_FUNC symbol which caused the problem?
Also I'm not quite convinced this patch is the right fix. For the 'pfunc_offset' calculation, I believe 'pfunc->sym.st_value' is the same value as 'pfunc->offset' -- they both represent the function's section offset. So it's basically adding pfunc->offset to itself, right? Is that intentional?
Any chance this patch fixes your issue? If not, the above readelf would help me understand it more.
diff --git a/tools/objtool/check.c b/tools/objtool/check.c index 5e5388a38e2a..4f30a763a4e3 100644 --- a/tools/objtool/check.c +++ b/tools/objtool/check.c @@ -1344,6 +1344,10 @@ static int add_jump_table(struct objtool_file *file, struct instruction *insn, if (prev_offset && reloc->offset != prev_offset + 8) break; + /* Jump table relocs are always STT_SECTION: */ + if (reloc->sym->type != STT_SECTION) + break; + /* Detect function pointers from contiguous objects: */ if (reloc->sym->sec == pfunc->sec && reloc->addend == pfunc->offset)
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