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Re: [Qemu-ppc] [PATCH] linux-user: elf: mmap all the target-pages of hos

From: Shivaprasad G Bhat
Subject: Re: [Qemu-ppc] [PATCH] linux-user: elf: mmap all the target-pages of hostpage for data segment
Date: Tue, 28 Aug 2018 20:53:42 +0530
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2

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_PAGESTART(_v) ((_v) & \
-                                 ~(abi_ulong)(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) ? \
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,


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