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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Rob Landley <rob@landley.net>
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Date:   Wed, 13 May 2020 16:59:35 -0500
In-Reply-To: <f33135b7-caa2-94f3-7563-fab6a1f5da0f@landley.net> (Rob Landley's
        message of "Mon, 11 May 2020 14:10:23 -0500")
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Subject: Re: [PATCH 3/5] exec: Remove recursion from search_binary_handler
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Rob Landley <rob@landley.net> writes:

> On 5/11/20 9:33 AM, Eric W. Biederman wrote:
>> What I do see is that interp_data is just a parameter that is smuggled
>> into the call of search binary handler.  And the next binary handler
>> needs to be binfmt_elf for it to make much sense, as only binfmt_elf
>> (and binfmt_elf_fdpic) deals with BINPRM_FLAGS_EXECFD.
>
> The binfmt_elf_fdpic driver is separate from binfmt_elf for the same reason
> ext2/ext3/ext4 used to have 3 drivers: fdpic is really just binfmt_elf with the
> 4 main sections (text, data, bss, rodata) able to move independently of each
> other (each tracked with its own base pointer).
>
> It's kind of -fPIE on steroids, and various security people have sniffed at it
> over the years to give ASLR more degrees of freedom on with-MMU systems. Many
> moons ago Rich Felker proposed teaching the fdpic loader how to load normal ELF
> binaries so there's just the one loader (there's a flag in the ELF header to say
> whether the sections are independent or not).

Careful with your terminology.  ELF sections are for .o's For
executables ELF have segments.  And reading through the code it is the
program segments that are independently relocatable.

There is a flag but it is defined per architecture and I don't think one
of the architectures define it.

I looked at ARM and apparently with an MMU ARM turns fdpic binaries into
PIE executables.  I am not certain why.

The registers passed to the entry point are also different for both
cases.

I think it would have been nice if the fdpic support had used a
different ELF type, instead of a different depending on using a
different architecture.

All that aside the core dumping code looks to be essentially the same
between binfmt_elf.c and binfmt_elf_fdpic.c.  Do you think people would
be interested in refactoring binfmt_elf.c and binfmt_elf_fdpic.c so that
they could share the same core dumping code?

Eric