Re: [PATCH v3 09/19] unwind: Introduce sframe user space unwinding

On Fri, Nov 01, 2024 at 11:34:48AM -0700, Andrii Nakryiko wrote:
> 00200000-170ad000 r--p 00000000 07:01 5
> 172ac000-498e7000 r-xp 16eac000 07:01 5
> 49ae7000-49b8b000 r--p 494e7000 07:01 5
> 49d8b000-4a228000 rw-p 4958b000 07:01 5
> 4a228000-4c677000 rw-p 00000000 00:00 0
> 4c800000-4ca00000 r-xp 49c00000 07:01 5
> 4ca00000-4f600000 r-xp 49e00000 07:01 5
> 4f600000-5b270000 r-xp 4ca00000 07:01 5
>
> Sorry, I'm probably dense and missing something. But from the example
> process above, isn't this check violated already? Or it's two
> different things? Not sure, honestly.

It's hard to tell exactly what's going on, did you strip the file names?

The sframe limitation is per file, not per address space.  I assume
these are one file:

> 172ac000-498e7000 r-xp 16eac000 07:01 5

and these are another:

> 4c800000-4ca00000 r-xp 49c00000 07:01 5
> 4ca00000-4f600000 r-xp 49e00000 07:01 5
> 4f600000-5b270000 r-xp 4ca00000 07:01 5

Multiple mappings for a single file is fine, as long as they're
contiguous.

> > Actually I just double checked and even the kernel's ELF loader assumes
> > that each executable has only a single text start+end address pair.
> 
> See above, very confused by such assumptions, but I'm hoping we are
> talking about two different things here.

The "contiguous text" thing seems enforced by the kernel for
executables.  However it doesn't manage shared libraries, those are
mapped by the loader, e.g. /lib64/ld-linux-x86-64.so.2.

At a quick glance I can't tell if /lib64/ld-linux-x86-64.so.2 enforces
that.

> > There's no point in adding complexity to support some hypothetical.  I
> > can remove the printk though.
> 
> We are talking about fundamental things like format for supporting
> frame pointer-less stack trace capture. It will take years to adopt
> SFrame everywhere, so I think it's prudent to think a bit ahead beyond
> just saying "no real application should need more than 4GB text", IMO.

I don't think anybody is saying that...

-- 
Josh