Re: git pull request for tip/tracing/urgent

[Frederic Weisbecker]

On Wed, Feb 11, 2009 at 10:02:29AM +0100, Ingo Molnar wrote:
> 
> * Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
> 
> > > > 
> > > > 
> > > > It thought after the fixup section, the code would continue to rest of the C code.
> > > > Where would it go without the jmp?
> > > 
> > > To the next item the linker placed into the .fixup section.  And that 
> > > would jump back to the location for that fixup. Basically, what you have 
> > > is this:
> > > 
> > > (just picking random and factitious registers)
> > > 
> > > .section .text
> > > [...]
> > > L1:	mov	%a, %b
> > > L2:	cmp	%x, $1
> > > <continue code>
> > > 
> > > 
> > > <Someplace else>
> > > 
> > > .section .text
> > > [...]
> > > L3:	mov	%c, %d
> > > L4:	cmp	%x, $22
> > > [...]
> > > 
> > > .section .fixup
> > > [...]
> > > L5:	mov	$1, %x
> > > 	jmp L2
> > > L6:	mov	$22, %x
> > > 	jmp L4
> > > [...]
> > > 
> > > 
> > > .section __ex_table
> > > [...]
> > > .long	L1, L5
> > > .long	L3, L6
> > > [...]
> > > 
> > > 
> > > So when we take an exception at label L1, the page fault code will look 
> > > to see if it is OK, by doing a binary search of the exception table.
> > > When it finds the L1, L5 pair, it will then set up a return to the L5 
> > > label.
> > > 
> > > When the fault returns to L5, it loads that reg %x with $1 and jumps back 
> > > to L2, where it can see that it took a fault.
> > > 
> > > Now lets look at what happens when we do not have that jump back to L2. 
> > > Instead of going back to the original code, it will load $22 into %x and 
> > > jmp back to the wrong area. God knows what will happen then, since the 
> > > stack pointer thinks it is from where the original fault occurred.
> > 
> > 
> > Heh, that's fairly logic. Don't ask me why, but I did not imagine each
> > part of .fixup unified in a separate contiguous section (but what else can it be?...).
> > 
> > Thanks for your explanations :-)
> 
> This bit:
> 
>   	".section .fixup, \"ax\"\n"
>  	"4: movl $1, %[faulted]\n"
>  	"   jmp 3b\n"
>  	".previous\n"
> 
> Can be thought of as an 'embedded' or 'nested' section - the '.previous'
> directive jumps back to whatever section we were in before. This can be
> nested multiple times too:
> 
>    .section A
>    [...]
>      .section B
>      [...]
>         .section C
>         [...]
>         .previous
>      [...]
>      .previous
>    [...]
>    .previous
> 
> For whatever reason the interaction of the assembler with the linker and
> in particular linker scripts are one of the most undocumented areas of OSS.
> Does anyone know any good reference to start with? 
> 
> Something that explains the principles, how it all works, what the various
> section flags mean in practice, including details like dwarf2/CFI annotations.
> 
> I do not know about any coherent documentation in this area and as a result
> many developers shy away from this area, frequently mess it up if they have to
> touch it and generally treat it as voodoo.
> 
> 	Ingo

When I wrote this part, I used the following documentation:

http://tldp.org/LDP/khg/HyperNews/get/devices/exceptions.html

But yeah, the assembler/linker stuff documentation for gcc are not so much
documented.

There is the raw reference:
http://sourceware.org/binutils/docs/as/index.html

Concerning examples, practical cases, it is exploded on several parts on the web, some good
links can be found on http://asm.sourceforge.net/

Thanks for your explanation.
But I'm confused, if the .previous make a jump to the previous section, then it already
does what jmp 3b does right?

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