Re: [PATCH v2 2/7] y2038: Introduce __ASSUME_64BIT_TIME define

[Stepan Golosunov]

02.05.2019 в 15:04:18 +0000 Joseph Myers написал:
> On Tue, 30 Apr 2019, Lukasz Majewski wrote:
> 
> >  - The need for explicit clearing padding when calling syscalls (as to
> >    be better safe than sorry in the future - there was related
> >    discussion started by Stepan).
> 
> This really isn't a difficult question.  What it comes down to is whether 
> the Linux kernel, in the first release version with these syscalls (we 
> don't care about old -rc versions; what matters is the actual 5.1 
> release), ignores the padding.
> 
> If 5.1 *release* ignores the padding, that is part of the kernel/userspace 
> ABI, in accordance with the kernel principle of not breaking userspace.  
> Thus, it is something userspace can rely on, now and in the future.
> 
> If 5.1 release does not ignore the padding, syscall presence does not mean 
> the padding is ignored by the kernel and so glibc needs to clear padding.  
> Of course, it needs to clear padding in a *copy* of the value provided by 
> the user unless the glibc API in question requires the timespec value in 
> question to be in writable memory.
> 
> So, which is (or will be) the case in 5.1 release?  Padding ignored or 
> not?  If more complicated (ignored for some architectures / ABIs but not 
> for others, or depending on whether compat syscalls are in use), then say 
> so - give a precise description of the exact circumstances under which the 
> padding around a 32-bit tv_nsec will or will not be ignored by the kernel 
> on input from userspace.

In current linux git it looks like padding is correctly ignored in
32-bit kernels (because kernel itself has 32-bit tv_nsec there) but
the code to clear it on compat syscalls in 64-bit kernels seems to be
broken.

The patch to fix this is at

https://lore.kernel.org/lkml/20190429131951.471701-1-arnd@xxxxxxxx/

but it doesn't seem like it has reached Linus yet.


(Hmm.  I think that old ipc and socketcall syscalls in 32-bit kernels
are broken without that patch too.  They would try to read
__kernel_timespec when callers are passing old_timespec32.)