[Xen-changelog] [xen master] x86/string: fix memmove when size is 0

[patchbot@xxxxxxx]

commit 4a1d823f70ac8182cbc5a4bd3c76beeb952e9683
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Fri Oct 20 09:27:23 2017 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Oct 20 09:27:23 2017 +0200

    x86/string: fix memmove when size is 0
    
    ubsan in clang 5.0 complains with:
    
    (XEN) UBSAN: Undefined behaviour in string.c:50:28
    (XEN) pointer overflow:
    (XEN) addition of unsigned offset to ffff830000100000 overflowed to 
ffff8300000fffff
    [...]
    (XEN) Xen call trace:
    (XEN)    [<ffff82d0802dce0d>] ubsan.c#ubsan_epilogue+0xd/0xc0
    (XEN)    [<ffff82d0802de145>] __ubsan_handle_pointer_overflow+0xa5/0xe0
    (XEN)    [<ffff82d0803bf627>] memmove+0x67/0x80
    (XEN)    [<ffff82d080679f87>] page_alloc.c#bootmem_region_add+0x157/0x220
    (XEN)    [<ffff82d080679c66>] init_boot_pages+0x56/0x220
    (XEN)    [<ffff82d0806bcb9d>] __start_xen+0x2c2d/0x4b50
    (XEN)    [<ffff82d0802000f3>] __high_start+0x53/0x60
    
    This is due to memmove doing a n-1+addr when n is 0. This is harmless
    because the loop is bounded to 0. Fix this by returning earlier when n
    is 0.
    
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    [jb: add return value and unlikely()]
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    Release-acked-by: Julien Grall <julien.grall@xxxxxxxxxx>
---
 xen/arch/x86/string.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/xen/arch/x86/string.c b/xen/arch/x86/string.c
index cd85a38..e2f8463 100644
--- a/xen/arch/x86/string.c
+++ b/xen/arch/x86/string.c
@@ -39,6 +39,9 @@ void *(memmove)(void *dest, const void *src, size_t n)
 {
     long d0, d1, d2;
 
+    if ( unlikely(!n) )
+        return dest;
+
     if ( dest < src )
         return memcpy(dest, src, n);
 
--
generated by git-patchbot for /home/xen/git/xen.git#master

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