linux-kernel - [PATCH] elf: align AT

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Date:   Thu, 30 May 2019 00:37:08 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     akpm@...ux-foundation.org
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: [PATCH] elf: align AT_RANDOM bytes

AT_RANDOM content is always misaligned on x86_64:

	$ LD_SHOW_AUXV=1 /bin/true | grep AT_RANDOM
	AT_RANDOM:       0x7fff02101019

glibc copies first few bytes for stack protector stuff, aligned
access should be slightly faster.

Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
---

 fs/binfmt_elf.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -144,11 +144,15 @@ static int padzero(unsigned long elf_bss)
 #define STACK_ALLOC(sp, len) ({ \
 	elf_addr_t __user *old_sp = (elf_addr_t __user *)sp; sp += len; \
 	old_sp; })
+#define STACK_ALIGN(sp, align)	\
+	((typeof(sp))(((unsigned long)sp + (int)align - 1) & ~((int)align - 1)))
 #else
 #define STACK_ADD(sp, items) ((elf_addr_t __user *)(sp) - (items))
 #define STACK_ROUND(sp, items) \
 	(((unsigned long) (sp - items)) &~ 15UL)
 #define STACK_ALLOC(sp, len) ({ sp -= len ; sp; })
+#define STACK_ALIGN(sp, align)	\
+	((typeof(sp))((unsigned long)sp & ~((int)align - 1)))
 #endif
 
 #ifndef ELF_BASE_PLATFORM
@@ -217,6 +221,12 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
 			return -EFAULT;
 	}
 
+	/*
+	 * glibc copies first bytes for stack protector purposes
+	 * which are misaligned on x86_64 because strlen("x86_64") + 1 == 7.
+	 */
+	p = STACK_ALIGN(p, sizeof(long));
+
 	/*
 	 * Generate 16 random bytes for userspace PRNG seeding.
 	 */