On 17/09/2019 17:06, Dave Martin wrote:
> 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.
>
Well the reason if that I was equally (but wrongly on my side) convinced
to have to sum +16 before clearing the lower 0b1111.
My bad. I'm fixing in v7.
>> + 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
>
Cheers
Cristian
