linux-kernel - Re: [RFC, PATCH] Fixup COMPAT_VDSO to work with CONFIG

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Date:	Thu, 15 Mar 2007 20:03:10 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Jan Beulich <jbeulich@...ell.com>,
	Rusty Russell <rusty@...tcorp.com.au>, Andi Kleen <ak@...e.de>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Chris Wright <chrisw@...s-sol.org>,
	Andrew Morton <akpm@...l.org>,
	Linus Torvalds <torvalds@...l.org>,
	Virtualization Mailing List <virtualization@...ts.osdl.org>
Subject: Re: [RFC, PATCH] Fixup COMPAT_VDSO to work with CONFIG_PARAVIRT

Jeremy Fitzhardinge wrote:
> Zachary Amsden wrote:
>   
>> Invoke black magic to relocate the VDSO even when COMPAT_VDSO is enabled
>> by fixing up the ELF object.
>>   
>>     
>
> So does it actually work?  Can you boot the broken distros with this in
> place?
>   

Well testing that is not so fun.  I installed SUSE Pro 9.0, and strings 
on ld.so contains the magic at_sysinfo assert!  But it doesn't install 
TLS libraries, so I'll have to install them by hand.

In works - in theory.  Look, a puppy!

Scratchbox is rumored to produce the fabled assertion even on modern 
distros by installing its own toolchain which includes the dreaded glibc.

> Using sections is wrong; you should be going through the phdrs, and
> looking for PT_DYNAMIC for relocation.
>   

Will do.

> Does anyone expect the symbolic info to be correct?  It might be better
> to just stomp it so nobody gets any ideas.
>
> On the other hand, we don't want to break compatibility with anything...
>   

I'm playing safe.  Binary identical relocation to 0xffffe000 was my goal.

>> +		} else if (strcmp(secstrings+sechdrs[i].sh_name, ".dynamic") == 0) {
>> +			Elf32_Dyn *dyn = (void *)hdr + sechdrs[i].sh_offset;
>> +			int tag;
>> +			while ((tag = (++dyn)->d_tag) != DT_NULL)
>>   
>>     
>
> Um, no.
>   

Walk based on size instead?

>> +		} else if (strcmp(secstrings+sechdrs[i].sh_name, ".useless") == 0) {
>> +			/* This is demonic; see vsyscall.lds.S; it puts the
>> +			 * .got in a section named .useless */
>> +			uint32_t *got = (void *)hdr + sechdrs[i].sh_offset;
>> +			*got += VDSO_HIGH_BASE;
>> +		}
>>   
>>     
>
> This won't get relocated with one of the other relocations?  It's in the
> text phdr.
>   

Hmm, I can try that.  Thanks for the suggestions / fixes.

Zach
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