Re: [PATCH] linux: add support for uname26 personality

[Aurelien Jarno <aurelien at aurel32 dot net>]

On 2015-12-03 10:12, Florian Weimer wrote:
> On 12/03/2015 12:22 AM, Aurelien Jarno wrote:
> > When the kernel is running under the uname26 personality (for programs
> > that cannot handle "Linux 3.0"), it maps version 3.x to 2.6.40+x and
> > 4.x to 2.6.60+x. When the GNU libc is configured with --enable-kernel=3.x
> > or 4.x and uname26 personality is enabled, binaries get abort with a
> > "FATAL: kernel too old message", even if the kernel actually supports
> > such a binary.
> 
> I'm sorry, I don't think this is the correct way to handle this.  The
> reason is that
> 
> > +/* When the kernel is running under the uname26 personality (for programs
> > +   that cannot handle "Linux 3.0"), it maps version 3.x to 2.6.40+x and
> > +   4.x to 2.6.60+x.  We need to convert that back to the standard version
> > +   numbering to be able to compare versions.  */
> > +static int
> > +convert_from_uname26 (int version)
> > +{
> > +  if ((version & 0xffff00) == 0x020600)
> > +    {
> > +      /* 2.6.40+x to 3.x */
> > +      if ((version & 0xff) >= 60)
> > +	version += 0x020000 - 60;
> > +      /* 2.6.60+x to 4.x */
> > +      else if ((version & 0xff) >= 40)
> > +	version += 0x010000 - 40;
> > +    }
> > +
> > +  return version;
> > +}
> 
> â this function will have to be changed again for Linux 5.0.
> 
> A long-term solution would map the minimum required version hard-coded
> into libc to its 2.6 equivalent, and check that if the kernel reports a
> 2.6 version.  This is solves the forward compatibility issue because the
> 2.6 mapping for the minimum version is known at the time glibc is compiled.

So you mean that we should have for example two defines in our code, one
for the standard version and the other for the 2.6 version and that both
would be defined at compilation time. Correct?

That still means we need to do the conversion at compile time to be able
to define both values.

Aurelien

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                 http://www.aurel32.net