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

On 1/24/25 11:21 AM, Josh Poimboeuf wrote:
> On Fri, Jan 24, 2025 at 10:00:52AM -0800, Andrii Nakryiko wrote:
>> On Tue, Jan 21, 2025 at 6:32 PM Josh Poimboeuf <[email protected]> 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.
> 

Yes, in theory, it is allowed (as per the specification) to have an 
SFrame section with zero number of FDEs/FREs.  But since such a section 
will not be useful, I share the opinion that it makes sense to disallow 
it in the current unwinding contexts, for now (JIT usecase may change 
things later).

SFRAME_F_FRAME_POINTER flag is not being set currently by GAS/GNU ld at all.

>>> +               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.
> 

SFrame specification aims to keep SFrame header and SFrame FDE members 
at aligned boundaries in future versions.

Only SFrame FRE related accesses may have unaligned accesses.