Re: Concerns about SFrame viability for userspace stack walking

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Oct 30, 2025 at 12:50:42AM -0700, Fangrui Song wrote:
> An effective compact unwinding scheme needs to leverage ISA-specific properties.

Having 40-50 completely different unwinding schemes, one for each
architecture or even ISA subset, would be a complete nightmare.  Plus the
important property of DWARF is that it is easily extensible.  So, I think it
would be better to invent new DWARF DW_CFA_* arch specific opcodes which
would be a shorthand for the most common sequences of unwind info, or allow
the CIEs to define a library of DW_CFA_* sets perhaps with parameters which
would then be usable in the FDEs.  There are already some arch specific
opcodes, DW_CFA_GNU_window_save for SPARC and
DW_CFA_AARCH64_negate_ra_state_with_pc/DW_CFA_AARCH64_negate_ra_state for
AArch64, but if somebody took time to look through .eh_frame of many
binaries/libraries on several different distributions for particular arch
(so that there is no bias in what exact options those distros use etc.) and
found something that keeps repeating there commonly that could be shortened,
perhaps the assembler or linker could rewrite sequences of specific .cfi_*
directives into something equivalent but shorter once the extension opcodes
are added.  Though, there are only very few opcodes left, so taking them
should be done with great care and at least one should be left as a
multiplexer (single byte opcode followed by uleb128 code for further
operation + arguments).

	Jakub