Re: [musl] Dynamic linker segfault

[Rich Felker <dalias@libc.org>]

On Wed, Jan 05, 2022 at 09:56:25AM +0100, Florian Weimer wrote:
> * Rich Felker:
> 
> > This is a malformed program file not compatible with the machine page
> > size (4k). Arguably it should be detected as p_align < PAGESIZE -- in
> > a sense, p_align for LOAD segments is the maximum supported page size
> > for the program file, and machines not capable of providing a page
> > size that small can't map/run it. In theory the loader could allow
> > this if all the differences between segments satisfy the right
> > congruences and have matching permissions where the maps would
> > overlap, but I'm not sure that's useful.
> 
> We've been looking at this on the glibc side recently.  The use case is
> supporting large data alignments (greater than the kernel page size)
> while not pessimizing multi-page-size targets such as POWER and AArch64.

I'm not clear how it pessimizes these targets (beyond what's
fundamentally necessary) unless you're artificially aligning segment
contents on disk to a large alignment boundary to prevent over-mapping
(undermining separate-code for example). And if you're doing that, you
need the full alignment anyway to support machines with larger
hardware pagesize. Otherwise you'd get back the overmapping (and
unwanted perission exposure).

The only other thing I can think of is pessimizing ASLR by requiring
more alignment (throwing away a few bits of position entropy).

In any case, do you know if this test file is somehow related to that
work, or is it just a guess? It doesn't seem to be related to me since
it's essentially a "pageless" mapping setup.

Rich