[PATCH] RISC-V: gas: Change initial CFI operation from DW_CFA_def_cfa_register to DW_CFA_def_cfa

[Nelson Chu]

Thanks for helping, Andrew :-)  Approved and committed.

Nelson

On Fri, May 17, 2024 at 9:59 PM Andrew Burgess <aburgess@redhat.com> wrote:

> Sung-hun Kim <sfoon.kim@samsung.com> writes:
>
> > The DWARF specification (especially, DWARF4 and 5 [1,2]) states that
> > DW_CFA_def_cfa_register cannot be used as the first CFI operation.
> > It said DW_CFA_def_cfa_register as follows:
> >
> >   ... This operation is valid only if the current CFA rule is defined
> >   to use a register and offset.
> >
> > So, DW_CFA_def_cfa_register can be used after that other definition
> > operation such as DW_CFA_def_cfa is called. However, the current gas
> > code emits DW_CFA_def_cfa_register as an initial CFI operation for RISCV.
> >
> > In the libgcc, the unwinding function does not care about it, so it can
> > unwind the call stack. However, on the third party library such as
> > libunwindstack in Android, it causes a fatal error.
> >
> > This patch changes the initial CFI operation to DW_CFA_def_cfa with
> > offset 0. It works as same as the previous one, but it does not have
> > any limitation so it satisfies the DWARF spec. This change resolves
> > the compatibility issue while preserving the original behaviour.
> >
> > Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31733
>
> I'm not a binutils maintainer, so can't approve this patch.
>
> But, for what it's worth, I think this is the right change to make.
>
> Reviewed-By: Andrew Burgess <aburgess@redhat.com>
>
> Thanks,
> Andrew
>
> >
> > [1] DWARF4 specification, https://dwarfstd.org/doc/DWARF4.pdf
> > [2] DWARF5 specification, https://dwarfstd.org/doc/DWARF5.pdf
> >
> > Signed-off-by: Sung-hun Kim <sfoon.kim@samsung.com>
> > ---
> >  gas/config/tc-riscv.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/gas/config/tc-riscv.c b/gas/config/tc-riscv.c
> > index 1757ff6e9e2..8d749581c1d 100644
> > --- a/gas/config/tc-riscv.c
> > +++ b/gas/config/tc-riscv.c
> > @@ -5209,7 +5209,7 @@ RISC-V options:\n\
> >  void
> >  riscv_cfi_frame_initial_instructions (void)
> >  {
> > -  cfi_add_CFA_def_cfa_register (X_SP);
> > +  cfi_add_CFA_def_cfa (X_SP, 0);
> >  }
> >
> >  int
> > --
> > 2.25.1
>
>
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