Re: [PATCH 7/8] Add 32 bit VDSO time support for 32 bit kernel

[Stefani Seibold]

Am Montag, den 03.02.2014, 08:36 -0800 schrieb Andy Lutomirski:
> On Sun, Feb 2, 2014 at 11:44 PM, Stefani Seibold <stefani@xxxxxxxxxxx> wrote:
> > Am Sonntag, den 02.02.2014, 16:12 -0800 schrieb Andy Lutomirski:
> >> On Sun, Feb 2, 2014 at 1:39 PM, Stefani Seibold <stefani@xxxxxxxxxxx> wrote:
> >> > Am Sonntag, den 02.02.2014, 08:46 -0800 schrieb Andy Lutomirski:
> >> >> On Sun, Feb 2, 2014 at 3:27 AM,  <stefani@xxxxxxxxxxx> wrote:
> >> >> > From: Stefani Seibold <stefani@xxxxxxxxxxx>
> >> >> >
> >> >> > This patch add the time support for 32 bit a VDSO to a 32 bit kernel.
> >> >>
> >> >> [...]
> >> >>
> >> >> Can you address the review comments from last time around?  For
> >> >> example, this still seems to have redundant vvar and hpet mappings, it
> >> >> doesn't use the VVAR macro, it moves the 32-bit compat vDSO, etc.
> >> >>
> >> >
> >> > I will address the compat VDSO issue.
> >> >
> >> > But the VVAR macro will be not a part of this patch set. If you depend
> >> > on this, feel free to create one. From my point of view this is not
> >> > feasible without a macro hacking, because the address accessing the vvar
> >> > area differs in kernel and VDSO user mode.
> >>
> >> Sorry, but "I will make the code messier for no apparent reason and I
> >> will not offer to fix it in the same series" gets my NAK.
> >>
> >> Hint: I'm talking about two or three lines of code in vvar.h.
> >>
> >
> > A hint back: if you threat me with a NAK for a requested code sequence
> > which currently no user, this is far away from professional. I am not
> > your trainee.
> >
> > BTW: If it is so easy, send me the two or three lines and i will merge
> > it ;-)
> 
> Something to the effect of:
> 
> #elif defined(BUILD_VDSO32)
> #define VVAR(name) (*vvar_ ## name)
> #endif
> 
> Should do the trick.

You are wrong... 

#ifdef BUILD_VDSO32

#define DECLARE_VVAR(offset, type, name) \
	extern type vvar_ ## name __attribute__((visibility("hidden")));

#define VVAR(name) (vvar_ ## name)

#else

/* Base address of vvars.  This is not ABI. */
#ifdef CONFIG_X86_64
#define VVAR_ADDRESS (-10*1024*1024 - 4096)
#else
extern char __vvar_page;

#define VVAR_ADDRESS (&__vvar_page)
#endif

This would do the trick!

But for 64 bit to 32 bit conversation layer in vclock_gettime.c there is
still a

struct arch_vsyscall_gtod_data arch_vvar_vsyscall_gtod_data
	__attribute__((visibility("hidden")));
#define gtod (&arch_vvar_vsyscall_gtod_data)

needed, because vvar_vsyscall_gtod_data is the 32 bit version, which
would result in incorrect access of the struct members. So my code can't
use this VVAR macro.

> 
> >
> >> >
> >> > I also see no redundant mapping. There are two modes, one is the map of
> >> > the kernel area the other maps the VDSO into the user space area. This
> >> > is exactly the behaviour of the origin VDSO implementation.
> >>
> >> No.
> >>
> >> In your series there are *three* mappings.  There are:
> >>
> >>  - The linear mapping that the kernel loader sets up (the writable
> >> mapping used in the kernel).  This is implicit and, of course, fine.
> >>  - There's the fixmap page, which aliases the normal kernel mapping at
> >> a fixed address with the user, ro, and nx attributes.  The 64-bit vDSO
> >> uses that mapping.  See vdso.h -- it's all arranged pretty clearly.
> >> Your code, for no discernible reason, sets up a fixmap entry on
> >> *32-bit* kernels.
> >>  - The vma that you're setting up adjacent to the actual vdso text.
> >> This is what you are using.
> >>
> >> Please choose *one* user-readable mapping for the 32-bit vdso and
> >> stick with it.  If the 64-bit vdso can use it to and userspace doesn't
> >> break, even better.  But a pointless set of extra fixmap entries is
> >> not okay.
> >>
> >
> > Again: I wrote that there are two modes for a 32 bit kernel and
> > therefore there are two mappings at the same time. Since there are both
> > ways available in a 32 bit kernel via the vdso32= kernel parameter, both
> > must be supported.
> >
> > Due the lack of a real fixmap for a 32 bit kernel (FIXADDR_TOP is a
> > variable), the HPET and VVAR Page can only relative addressed. So this
> > pages must located before or after the VDSO.
> >
> > This is why i need to setup this pages into the fixmap area, this is the
> > compat mode "vdso32=2".
> >
> > For "vdso32=1" i need to map the VDSO Page together with the HPET and
> > VVAR into the user space.
> >
> > For compability reasons both mappings are required.
> 
> Not at the same time, and I don't think you've guarded the
> map_vsyscall call with a check for compat mode.
> 

I see no benefit to check the compat mode in map_vsyscall. The best way
is always map the VVAR page, because it is less error prone. In your
case tt would be also necessary not map the HPET page in non compat mode
too. So why bloat the code?

- Stefani


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