Re: [RFC PATCH] objtool: Skip unannotated intra-function call warning for bl+mflr pattern

[Christophe Leroy]

Le 24/02/2025 à 08:15, Christophe Leroy a écrit :
> Le 19/02/2025 à 17:20, Sathvika Vasireddy a écrit :
> > Architectures like PowerPC use a pattern where the compiler generates a
> > branch-and-link (bl) instruction that targets the very next instruction,
> > followed by loading the link register (mflr) later. This pattern appears
> > in the code like:
> >
> >   bl .+4
> >   li r5,0
> >   mflr r30
>
> What compiler do you use ? Is it a very old version of GCC ?

Oh, I see that this is a report on a projet version of clang ? compiler: 
clang version 21.0.0git

Then I guess the bug needs to be fixed in Clang, not in the kernel.

> That sequence is not correct and should never be used by modern 
> compilers. It should be bcl 20,31,+4 instead.
>
> All such hand writen sequences have been removed from kernel assembly, 
> see commit c974809a26a1 ("powerpc/vdso: Avoid link stack corruption in 
> __get_datapage()") for details
>
>
> > Objtool currently warns about this as an "unannotated intra-function
> > call" because find_call_destination() fails to find any symbol at the
> > target offset. Add a check to skip the warning when a branch targets
> > the immediate next instruction in the same function.
>
> I think this should be done in arch_decode_instruction(), just set insn- 
>  >type to INSN_OTHER when you see bl .+4
>
> Something like:
>
> diff --git a/tools/objtool/arch/powerpc/decode.c b/tools/objtool/arch/ 
> powerpc/decode.c
> index 53b55690f320..ca264c97ee8d 100644
> --- a/tools/objtool/arch/powerpc/decode.c
> +++ b/tools/objtool/arch/powerpc/decode.c
> @@ -55,7 +55,9 @@ int arch_decode_instruction(struct objtool_file *file, 
> const struct section *sec
>
>       switch (opcode) {
>       case 18: /* b[l][a] */
> -        if ((ins & 3) == 1) /* bl */
> +        if (ins == 0x48000005)    /* bl .+4 */
> +            typ = INSN_OTHER;
> +        else if ((ins & 3) == 1) /* bl */
>               typ = INSN_CALL;
>
>           imm = ins & 0x3fffffc;
>
>
>
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > Closes: https://lore.kernel.org/oe-kbuild-all/202502180818.XnFdv8I8- 
> > lkp@xxxxxxxxx/
> > Signed-off-by: Sathvika Vasireddy <sv@xxxxxxxxxxxxx>
> > ---
> >   tools/objtool/check.c | 6 ++++++
> >   1 file changed, 6 insertions(+)
> >
> > diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> > index 753dbc4f8198..3f7cf2c917b5 100644
> > --- a/tools/objtool/check.c
> > +++ b/tools/objtool/check.c
> > @@ -1613,6 +1613,7 @@ static struct symbol 
> > *find_call_destination(struct section *sec, unsigned long o
> >    */
> >   static int add_call_destinations(struct objtool_file *file)
> >   {
> > +       struct instruction *next_insn;
> >          struct instruction *insn;
> >          unsigned long dest_off;
> >          struct symbol *dest;
> > @@ -1625,6 +1626,11 @@ static int add_call_destinations(struct 
> > objtool_file *file)
> >                  reloc = insn_reloc(file, insn);
> >                  if (!reloc) {
> >                          dest_off = arch_jump_destination(insn);
> > +
> > +                       next_insn = next_insn_same_func(file, insn);
> > +                       if (next_insn && dest_off == next_insn->offset)
> > +                               continue;
> > +
> >                          dest = find_call_destination(insn->sec, 
> > dest_off);
> >
> >                          add_call_dest(file, insn, dest, false);
> > --
> > 2.39.3