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

[Josh Poimboeuf]

On Fri, Jan 24, 2025 at 03:13:40PM -0500, Steven Rostedt wrote:
> On Fri, 24 Jan 2025 11:21:59 -0800
> Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> 
> > > 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.
> 
> Hmm, could that be useful for implementing a way to dynamically grow or
> shrink an sframe because of jits? I'm just thinking about placeholders or
> sohething.

Maybe?

I was thinking the kernel would have sframe_section placeholders for JIT
code sections, so when sframe_find() retrieves the struct for a given
IP, it sees the JIT flag is set along with a pointer to the in-memory
shared "sframe section", then goes to read that to get the corresponding
sframe entry (insert erratic hand waving).

It's still early days but it's quite possible the in-memory "sframe
section" formats might end up looking pretty different from the .sframe
file section spec.

-- 
Josh