Re: 2.6.29-rc3-git6: Reported regressions from 2.6.28

[Ingo Molnar]

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> > Bug-Entry	: http://bugzilla.kernel.org/show_bug.cgi?id=12505
> > Subject		: 2.6.29-rc1 Firefox crashing on page load
> > Submitter	: Justin Madru <jdm64@xxxxxxxxx>
> > Date		: 2009-01-16 20:56 (20 days old)
> > First-Bad-Commit: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=4217458dafaa57d8e26a46f5d05ab8c53cf64191
> > References	: http://marc.info/?l=linux-kernel&m=123213941914274&w=4
> > Handled-By	: Justin P. Mattock <justinmattock@xxxxxxxxx>
> 
> Heh. That commit you point to is very innocuous.
> 
> Commit 4217458dafaa57d8e26a46f5d05ab8c53cf64191 is a pure cleanup, and it 
> _does_ make the code look nicer, but while it looks lik a total no-op, it 
> actually has a very subtle behavioural change: it makes gcc think that it 
> owns the whole argument stack. Which is _not_ true of 'asmlinkage' 
> functions, but we've never been able to tell gcc to keep its grubby hands 
> off our stack.
> 
> So I suspect that gcc inlines the function and then creates a function 
> call that re-uses the "struct pt_regs" on the stack as the argument area 
> too. Which corrupts the "pt_regs", and then we return to user space with 
> some random register contents.

yes - i tracked it down back then, just the regression entry didnt get 
updated.

> So I think we need to just revert it. Ingo, Hiroshi?

should be fixed upstream by:

 552b8aa: Revert "x86: signal: change type of paramter for sys_rt_sigreturn()"

the revert commit is below with explaination - also with a Tested-by tag. 
The revert is not a pure revert - it also adds a comment to preserve this 
circumstance cleanly.

We first had a few fruitless attempts to annotate it in some clear fashion 
(the existing tailcall-avoidance macros we have didnt suit this case) - then 
went for the revert because none of the variants that told gcc not to screw 
up actually made the code better than it was originally.

	Ingo

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