Because AT_RANDOM array is put on stack after AT_PLATFORM and AT_BASE_PLATFORM strings, it's aligned on a 1 byte boundary: on x86: $ for i in 1 2 3 ; do LD_SHOW_AUXV=1 ./bin32 | grep AT_RANDOM ; done AT_RANDOM: 0xffb91ffb AT_RANDOM: 0xffcd60eb AT_RANDOM: 0xffd7d99b on x86_64: $ for i in 1 2 3 ; do LD_SHOW_AUXV=1 ./bin64 | grep AT_RANDOM ; done AT_RANDOM: 0x7ffe3ce7d559 AT_RANDOM: 0x7ffeb9ecd2b9 AT_RANDOM: 0x7ffd37cebd49 Unlike AT_ENTROPY1 and AT_ENTROPY2, AT_ENTROPY, or AT_RANDOM1 and AT_RANDOM2 proposals[1][2][3] which would be 32bits or 64bits integers, kernel never promise AT_RANDOM array to be aligned[4]. But having a 16bytes array not aligned might not be optimal. Userspace could benefit from an aligned array. As the AT_PLATFORM and AT_BASE_PLATFORM are asciiz C strings, they don't need to be aligned, thus they can be put after AT_RANDOM array. As AT_RANDOM array is first put on stack, it's alignment should match the one set by arch_align_stack(). [1] https://lore.kernel.org/lkml/4675C678.3080807@xxxxxxxxxx/ [2] https://lore.kernel.org/lkml/467948F5.3010709@xxxxxxxxxx/ [3] https://sourceware.org/ml/libc-alpha/2008-10/msg00013.html [1] https://lore.kernel.org/lkml/20081003001616.GN10632@xxxxxxxxxxx/ Link: https://lore.kernel.org/lkml/cover.1560423331.git.ydroneaud@xxxxxxxxxx Signed-off-by: Yann Droneaud <ydroneaud@xxxxxxxxxx> --- fs/binfmt_elf.c | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-) diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c index 87f0c8a21350..cfcc01fff4ae 100644 --- a/fs/binfmt_elf.c +++ b/fs/binfmt_elf.c @@ -219,6 +219,14 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec, p = arch_align_stack(p); + /* + * Generate 16 random bytes for userspace PRNG seeding. + */ + get_random_bytes(k_rand_bytes, sizeof(k_rand_bytes)); + u_rand_bytes = elf_stack_alloc(&p, sizeof(k_rand_bytes)); + if (__copy_to_user(u_rand_bytes, k_rand_bytes, sizeof(k_rand_bytes))) + return -EFAULT; + /* * If this architecture has a platform capability string, copy it * to userspace. In some cases (Sparc), this info is impossible @@ -247,14 +255,6 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec, return -EFAULT; } - /* - * Generate 16 random bytes for userspace PRNG seeding. - */ - get_random_bytes(k_rand_bytes, sizeof(k_rand_bytes)); - u_rand_bytes = elf_stack_alloc(&p, sizeof(k_rand_bytes)); - if (__copy_to_user(u_rand_bytes, k_rand_bytes, sizeof(k_rand_bytes))) - return -EFAULT; - /* Create the ELF interpreter info */ elf_info = (elf_addr_t *)current->mm->saved_auxv; /* update AT_VECTOR_SIZE_BASE if the number of NEW_AUX_ENT() changes */ -- 2.21.0