Re: [Qemu-ppc] [PATCH] linux-user: elf: mmap all the target-pages of hostpage for data segment

[Shivaprasad G Bhat]

On 08/27/2018 06:55 PM, Laurent Vivier wrote:
> Le 27/08/2018 à 14:37, Shivaprasad G Bhat a écrit :
> > If the hostpage size is greater than the TARGET_PAGESIZE, the
> > target-pages of size TARGET_PAGESIZE are marked valid only till the
> > length requested during the elfload. The glibc attempts to consume unused
> > space in the last page of data segment(__libc_memalign() in
> > elf/dl-minimal.c). The GLRO(dl_pagesize) is actually the host pagesize as
> > set in the auxillary vectors. So, there is no explicit mmap request for
> > the remaining target-pages on the last hostpage. The glibc assumes that
> > particular space as available and subsequent attempts to use
> > those addresses lead to crash as the target_mmap has not marked them valid
> > for those target-pages.
> >
> > The issue is seen when trying to chroot to 16.04-x86_64 ubuntu on a PPC64
> > host where the fork fails to access the thread_id as it is allocated on a
> > page not marked valid. The recent glibc doesnt have checks for thread-id in
> > fork, but the issue can manifest somewhere else, none the less.
> >
> > The fix here is to map all the target-pages of the hostpage during the
> > ELF load for data segment to allow the glibc for proper consumption.
> >
> > Signed-off-by: Shivaprasad G Bhat <address@hidden>
> > ---
> >   linux-user/elfload.c |   24 +++++++++++++++++-------
> >   1 file changed, 17 insertions(+), 7 deletions(-)
> >
> > diff --git a/linux-user/elfload.c b/linux-user/elfload.c
> > index 8638612aec..1d86034c8d 100644
> > --- a/linux-user/elfload.c
> > +++ b/linux-user/elfload.c
> > @@ -1438,9 +1438,17 @@ struct exec
> >   
> >   /* Necessary parameters */
> >   #define TARGET_ELF_EXEC_PAGESIZE TARGET_PAGE_SIZE
> > -#define TARGET_ELF_PAGESTART(_v) ((_v) & \
> > -                                 ~(abi_ulong)(TARGET_ELF_EXEC_PAGESIZE-1))
> > -#define TARGET_ELF_PAGEOFFSET(_v) ((_v) & (TARGET_ELF_EXEC_PAGESIZE-1))
> > +#define TARGET_ELF_PAGESTART(_v, _s) \
> > +        ((TARGET_ELF_EXEC_PAGESIZE > _s) ? \
> > +         (_v) & ~(abi_ulong)(TARGET_ELF_EXEC_PAGESIZE - 1) : \
> > +         (_v) & ~(abi_ulong)(_s - 1));
> > +#define TARGET_ELF_PAGEOFFSET(_v, _s) \
> > +        ((TARGET_ELF_EXEC_PAGESIZE > _s) ? \
> > +         (_v) & (TARGET_ELF_EXEC_PAGESIZE - 1) : \
> > +         (_v) & (_s - 1));
> > +#define TARGET_ELF_PAGELENGTH(_v, _s) \
> > +        ((TARGET_ELF_EXEC_PAGESIZE > _s) ? \
> > +         TARGET_PAGE_ALIGN(_v) : HOST_PAGE_ALIGN(_v));
> I think it's only possible if the PT_LOAD p_align value is greater or
> equal to qemu_host_page_size.
>
> See 33143c446e ("linux-user: fix ELF load alignment error").
>
> You could check this with qemu-s390x or qemu-arm on ppc64 host.
Ah, right! I should have added the extra conditional to check the 
p_align with qemu_host_page_mask
along with the existing ones.

Posted the v2 accordingly.

Thanks and Regards,
Shivaprasad

> Thanks,
> Laurent