Re: [PATCH v4 17/39] unwind_user/sframe: Add support for reading .sframe headers

Josh Poimboeuf
2025-01-24T19:22:02Z

On Fri, Jan 24, 2025 at 10:00:52AM -0800, Andrii Nakryiko wrote:
> On Tue, Jan 21, 2025 at 6:32 PM Josh Poimboeuf <jpoimboe@kernel.org> wrote:
> > +static inline int sframe_add_section(unsigned long sframe_start, unsigned long sframe_end, unsigned long text_start, unsigned long text_end) { return -ENOSYS; }
>
> nit: very-very long, wrap it?

That was intentional as it's just an empty stub, but yeah, maybe 160
chars is a bit much.

> > + if (shdr.preamble.magic != SFRAME_MAGIC ||
> > + shdr.preamble.version != SFRAME_VERSION_2 ||
> > + !(shdr.preamble.flags & SFRAME_F_FDE_SORTED) ||
>
> probably more a question to Indu, but why is this sorting not
> mandatory and part of SFrame "standard"? How realistically non-sorted
> FDEs would work in practice? Ain't nobody got time to sort them just
> to unwind the stack...

No idea...

> > + if (!shdr.num_fdes || !shdr.num_fres) {
>
> given SFRAME_F_FRAME_POINTER in the header, is it really that
> nonsensical and illegal to have zero FDEs/FREs? Maybe we should allow
> that?

It would seem a bit silly to create an empty .sframe section just to set
that SFRAME_F_FRAME_POINTER bit. Regardless, there's nothing the kernel
can do with that.

> > + dbg("no fde/fre entries\n");
> > + return -EINVAL;
> > + }
> > +
> > + header_end = sec->sframe_start + SFRAME_HEADER_SIZE(shdr);
> > + if (header_end >= sec->sframe_end) {
>
> if we allow zero FDEs/FREs, header_end == sec->sframe_end is legal, right?

I suppose so, but again I'm not seeing any reason to support that.

> > + dbg("header doesn't fit in section\n");
> > + return -EINVAL;
> > + }
> > +
> > + num_fdes = shdr.num_fdes;
> > + fdes_start = header_end + shdr.fdes_off;
> > + fdes_end = fdes_start + (num_fdes * sizeof(struct sframe_fde));
> > +
> > + fres_start = header_end + shdr.fres_off;
> > + fres_end = fres_start + shdr.fre_len;
> > +
>
> maybe use check_add_overflow() in all the above calculation, at least
> on 32-bit arches this all can overflow and it's not clear if below
> sanity check detects all possible overflows

Ok, I'll look into it.

> > +struct sframe_preamble {
> > + u16 magic;
> > + u8 version;
> > + u8 flags;
> > +} __packed;
> > +
> > +struct sframe_header {
> > + struct sframe_preamble preamble;
> > + u8 abi_arch;
> > + s8 cfa_fixed_fp_offset;
> > + s8 cfa_fixed_ra_offset;
> > + u8 auxhdr_len;
> > + u32 num_fdes;
> > + u32 num_fres;
> > + u32 fre_len;
> > + u32 fdes_off;
> > + u32 fres_off;
> > +} __packed;
> > +
> > +struct sframe_fde {
> > + s32 start_addr;
> > + u32 func_size;
> > + u32 fres_off;
> > + u32 fres_num;
> > + u8 info;
> > + u8 rep_size;
> > + u16 padding;
> > +} __packed;
>
> I couldn't understand from SFrame itself, but why do sframe_header,
> sframe_preamble, and sframe_fde have to be marked __packed, if it's
> all naturally aligned (intentionally and by design)?..

Right, but the spec says they're all packed. Maybe the point is that
some future sframe version is free to introduce unaligned fields.

--
Josh