[PATCHv6 1/5] x86/mm: introduce arch_rnd() to compute 32/64 mmap rnd

[Dmitry Safonov]

To fix 32-bit mmap() syscall returning pointer higher than 4Gb in
64-bit binaries, two mmap bases will be used: one for mapping with
32-bit syscalls and another for 64-bit syscall.
To correctly place those two bases, introduce arch_rnd() function,
which will return the random factor independently of mmap_is_ia32().

Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Dmitry Safonov <dsafonov@xxxxxxxxxxxxx>
---
 arch/x86/mm/mmap.c | 26 ++++++++++++++------------
 1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
index 7940166c799b..f31ed7097d0b 100644
--- a/arch/x86/mm/mmap.c
+++ b/arch/x86/mm/mmap.c
@@ -55,6 +55,14 @@ static unsigned long stack_maxrandom_size(void)
 #define MIN_GAP (128*1024*1024UL + stack_maxrandom_size())
 #define MAX_GAP (TASK_SIZE/6*5)
 
+#ifdef CONFIG_COMPAT
+# define mmap32_rnd_bits  mmap_rnd_compat_bits
+# define mmap64_rnd_bits  mmap_rnd_bits
+#else
+# define mmap32_rnd_bits  mmap_rnd_bits
+# define mmap64_rnd_bits  mmap_rnd_bits
+#endif
+
 static int mmap_is_legacy(void)
 {
 	if (current->personality & ADDR_COMPAT_LAYOUT)
@@ -66,20 +74,14 @@ static int mmap_is_legacy(void)
 	return sysctl_legacy_va_layout;
 }
 
-unsigned long arch_mmap_rnd(void)
+static unsigned long arch_rnd(unsigned int rndbits)
 {
-	unsigned long rnd;
-
-	if (mmap_is_ia32())
-#ifdef CONFIG_COMPAT
-		rnd = get_random_long() & ((1UL << mmap_rnd_compat_bits) - 1);
-#else
-		rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);
-#endif
-	else
-		rnd = get_random_long() & ((1UL << mmap_rnd_bits) - 1);
+	return (get_random_long() & ((1UL << rndbits) - 1)) << PAGE_SHIFT;
+}
 
-	return rnd << PAGE_SHIFT;
+unsigned long arch_mmap_rnd(void)
+{
+	return arch_rnd(mmap_is_ia32() ? mmap32_rnd_bits : mmap64_rnd_bits);
 }
 
 static unsigned long mmap_base(unsigned long rnd)
-- 
2.11.1