RCU bug with v3.17-rc3 ?

[Russell King - ARM Linux]

On Mon, Oct 13, 2014 at 09:11:34AM +0000, David Laight wrote:
> From: Nathan Lynch
> > On 10/10/2014 11:25 AM, Russell King - ARM Linux wrote:
> > >
> > > Right, so GCC 4.8.{1,2} are totally unsuitable for kernel building (and
> > > it seems that this has been known about for some time.)
> > 
> > Looking at http://gcc.gnu.org/PR58854 it seems that all 4.8.x for x < 3
> > are affected, as well as 4.9.0.
> > 
> > > We can blacklist these GCC versions quite easily.  We already have GCC
> > > 3.3 blacklisted, and it's trivial to add others.  I would want to include
> > > some proper details about the bug, just like the other existing entries
> > > we already have in asm-offsets.c, where we name the functions that the
> > > compiler is known to break where appropriate.
> > 
> > Before blacklisting anything, it's worth considering that simple version
> > checks would break existing pre-4.8.3 compilers that have been patched
> > for PR58854.  It looks like Yocto and Buildroot issued releases with
> > patched 4.8.2 compilers well before the (fixed) 4.8.3 release.  I think
> > the most we can reasonably do without breaking some correctly-behaving
> > toolchains is to emit a warning.
> 
> Is it possible to compile a small code fragment and check the generated
> code for the bug?
> Possibly predicated on the broken version number to avoid false positives.

I don't see how - it looks like it requires an interrupt to occur at an
opportune moment to provoke the function to fail.  The alternative would
be to parse the assembly generated by the compiler to determine how it
is dealing with the stack.

I think the only viable solution here is that:

1. We blacklist the bad compiler versions outright in the kernel.
2. We /consider/ a testing a preprocessor symbol which when present
   indicates that these versions are fixed and should not be blacklisted.

The argument for (2) is that /if/ distros want to patch their compilers
to fix the problem, they /also/ have the ability to patch their compilers
to make them identifyable, and that is a far more reliable solution than
trying to parse the assembly output from multiple different GCC versions.

Remember, it's the distro's choice to fix these buggy compilers, so the
onus is on _them_ to deal with the mess they've created by doing so.

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