Re: [PATCH v2] ld.so: Handle read-only dynamic section gracefully [BZ #28340]

[Siddhesh Poyarekar via Libc-alpha <libc-alpha@sourceware.org>]

On 9/15/21 5:48 PM, Florian Weimer via Libc-alpha wrote:
> * Siddhesh Poyarekar:
> 
>> On 9/15/21 4:10 PM, Florian Weimer wrote:
>>> As far as I can tell, this does not check whether the DYNAMIC segment is
>>> actually covered by a read-write LOAD segment.  I wonder how much value
>>> this imperfect check has.
>>
>> I reckon there's less value in trying to make this corner case work; I
>> can't think of a reason for someone (outside of the VDSO use case) to
>> do this on purpose.  Read-only DYNAMIC segments in ET_DYN objects
>> shouldn't be a supported use case.
> 
> Sure, but the corresponding LOAD segment is read-only as well, that's
> what actually matters.

I don't understand.  Why isn't the fact that the DYNAMIC segment is 
read-only sufficient to bail out with at error?  Or to rephrase, what 
valid use case would we be supporting by checking flags on the LOAD 
segment containing the .dynamic section to check for write permission?

I know that the crash wouldn't happen if the LOAD was read-write, but I 
don't know if the additional check covers a valid use case.

> And they do not want multiple LOAD segments because it can fit all on
> the same page.  We would end up with an RWX segment if we made this
> single LOAD segment writable, which is probably even more confusing.

Right.

> So I guess the status quo is probably best.

I don't know about that :)

> Maybe someone else can comment on the additional detection.  In the
> past, we said that crashes on corrupt ELF files weren't something we
> cared about, but I increasingly see th value of good diagnostics for
> common special cases.

This seems like a situation where we can add an inexpensive check and 
not crash.

> (See also the discussion about DSOs without entry points.)

I need to collect my thoughts about that.  I had run into it last year 
and I need to see if I had written something down then.  I'd definitely 
like to know if there is a historical perspective to setting .text as 
the entry point by default.

Siddhesh