On Tue, Sep 10, 2019 at 01:31:06pm +0100, Cristian Marussi wrote:
> Add a simple fake_sigreturn testcase which builds a ucontext_t with a bad
> magic header and place it onto the stack. Expects a SIGSEGV on test PASS.
>
> Introduce a common utility assembly trampoline function to invoke a
> sigreturn while placing the provided sigframe at wanted alignment and
> also an helper to make space when needed inside the sigframe reserved
> area.
>
> Signed-off-by: Cristian Marussi <cristian.marussi@xxxxxxx>
> ---
> v5 --> v6
> - fake_sigreturn: cleaned up, avoiding excessive SP misalignments
> - fake_sigreturn: better formatting and prologue
> - get_starting_header: cleand up and commented
> - avoid timeout on failure
[...]
> diff --git a/tools/testing/selftests/arm64/signal/signals.S b/tools/testing/selftests/arm64/signal/signals.S
> new file mode 100644
> index 000000000000..e670f8f2c8de
> --- /dev/null
> +++ b/tools/testing/selftests/arm64/signal/signals.S
> @@ -0,0 +1,64 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/* Copyright (C) 2019 ARM Limited */
> +
> +#include <asm/unistd.h>
> +
> +.section .rodata, "a"
> +call_fmt:
> + .asciz "Calling sigreturn with fake sigframe sized:%zd at SP @%08lX\n"
> +
> +.text
> +
> +.globl fake_sigreturn
> +
> +/* fake_sigreturn x0:&sigframe, x1:sigframe_size, x2:misalign_bytes */
> +fake_sigreturn:
> + stp x29, x30, [sp, #-16]!
> + mov x29, sp
> +
> + mov x20, x0
> + mov x21, x1
> + mov x22, x2
> +
> + /* create space on the stack for fake sigframe 16 bytes-aligned */
> + add x0, x21, x22
> + add x0, x0, #16
> + bic x0, x0, #15 /* round_up(sigframe_size + misalign_bytes, 16) */
If I've figured this out right, x0 as computed here actually looks
like round_up(sigframe_size + misalign_bytes + 1, 16) - 1.
(n + (m - 1)) & ~(m - 1) is the classic way to round up when m is a
power of 2.
That's why I originally suggested to add 15. Your code works, but I
think it always allocates at least one byte more than needed (?)
This is not a huge deal, and better than allocating one byte to few,
but it would be good to understand whether this behaviour was
intentional or not.
> + sub sp, sp, x0
> + add x23, sp, x22 /* new sigframe base with misaligment if any */
> +
> + ldr x0, =call_fmt
> + mov x1, x21
> + mov x2, x23
> + bl printf
> +
> + /* memcpy the provided content, while still keeping SP aligned */
> + mov x0, x23
> + mov x1, x20
> + mov x2, x21
> + bl memcpy
> +
> + /*
> + * Here saving a last minute SP to current->token acts as a marker:
> + * if we got here, we are successfully faking a sigreturn; in other
> + * words we are sure no bad fatal signal has been raised till now
> + * for unrelated reasons, so we should consider the possibly observed
> + * fatal signal like SEGV coming from Kernel restore_sigframe() and
> + * triggered as expected from our test-case.
> + * For simplicity this assumes that current field 'token' is laid out
> + * as first in struct tdescr
> + */
> + ldr x0, current
> + str x23, [x0]
> + /* finally move SP to misaligned address...if any requested */
> + mov sp, x23
> +
> + mov x8, #__NR_rt_sigreturn
> + svc #0
> +
> + /*
> + * Above sigreturn should not return...looping here leads to a timeout
> + * and ensure proper and clean test failure, instead of jumping around
> + * on a potentially corrupted stack.
> + */
> + b .
[...]
> diff --git a/tools/testing/selftests/arm64/signal/testcases/testcases.c b/tools/testing/selftests/arm64/signal/testcases/testcases.c
> index 1914a01222a1..e3521949b800 100644
> --- a/tools/testing/selftests/arm64/signal/testcases/testcases.c
> +++ b/tools/testing/selftests/arm64/signal/testcases/testcases.c
> @@ -148,3 +148,49 @@ bool validate_reserved(ucontext_t *uc, size_t resv_sz, char **err)
>
> return true;
> }
> +
> +/*
> + * This function walks through the records inside the provided reserved area
> + * trying to find enough space to fit @need_sz bytes: if not enough space is
> + * available and an extra_context record is present, it throws away the
> + * extra_context record.
> + *
> + * It returns a pointer to a new header where it is possible to start storing
> + * our need_sz bytes.
> + *
> + * @shead: points to the start of reserved area
> + * @need_sz: needed bytes
> + * @resv_sz: reserved area size in bytes
> + * @offset: if not null, this will be filled with the offset of the return
> + * head pointer from @shead
> + *
> + * @return: pointer to a new head where to start storing need_sz bytes, or
> + * NULL if space could not be made available.
> + */
That's much clearer now, thanks for that.
> +struct _aarch64_ctx *get_starting_head(struct _aarch64_ctx *shead,
> + size_t need_sz, size_t resv_sz,
> + size_t *offset)
> +{
[...]
Cheers
---Dave
