Re: [RFC PATCH v2 1/2] rseq: x86: implement abort-at-ip extension

Mathieu Desnoyers
2022-01-11T17:43:09Z

----- On Jan 11, 2022, at 6:05 AM, Christian Brauner brauner@kernel.org wrote:

> On Mon, Jan 10, 2022 at 12:16:10PM -0500, Mathieu Desnoyers wrote:
>> Allow rseq critical section abort handlers to optionally figure out at
>> which instruction pointer the rseq critical section was aborted.
>>
>> This allows implementing rseq critical sections with loops containing
>> the commit instruction, for which having the commit as last instruction
>> of the sequence is not possible. This is useful to implement adaptative
>> mutexes aware of preemption in user-space. (see [1])
>>
>> This also allows implementing rseq critical sections with multiple
>> commit steps, and use the abort-at-ip information to figure out what
>> needs to be undone in the abort handler.
>>
>> Introduce the RSEQ_FLAG_QUERY_ABORT_AT_IP rseq system call flag. This
>> lets userspace query whether the kernel and architecture supports the
>> abort-at-ip rseq extension.
>>
>> Only provide this information for rseq critical sections for which the
>> rseq_cs descriptor has the RSEQ_CS_FLAG_ABORT_AT_IP flag set. Abort
>> handlers for critical sections with this flag set need to readjust the
>> stack pointer. The abort-at-ip pointer is populated by the kernel on
>> the top of stack on abort. For x86-32, the abort handler of an
>> abort-at-ip critical section needs to add 4 bytes to the stack pointer.
>> For x86-64, the abort hanler needs to add 136 bytes to the stack
>> pointer: 8 bytes to skip the abort-at-ip value, and 128 bytes to skip
>> the x86-64 redzone for leaf functions.
>>
>> [1]
>> https://github.com/compudj/rseq-test/blob/adapt-lock-abort-at-ip/test-rseq-adaptative-lock.c#L80
>>
>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>> ---
>> Changes since v1:
>> - Use top of stack to place abort-at-ip value rather than ecx/rcx
>> register,
>> - Skip redzone on x86-64.
>> ---
>> arch/x86/include/asm/ptrace.h | 5 +++++
>> arch/x86/kernel/ptrace.c | 12 ++++++++++++
>> include/uapi/linux/rseq.h | 4 ++++
>> kernel/rseq.c | 28 ++++++++++++++++++++++++++++
>> 4 files changed, 49 insertions(+)
>>
>> diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
>> index 703663175a5a..c96eb2448110 100644
>> --- a/arch/x86/include/asm/ptrace.h
>> +++ b/arch/x86/include/asm/ptrace.h
>> @@ -387,5 +387,10 @@ extern int do_set_thread_area(struct task_struct *p, int
>> idx,
>> # define do_set_thread_area_64(p, s, t) (0)
>> #endif
>>
>> +#ifdef CONFIG_RSEQ
>> +# define RSEQ_ARCH_HAS_ABORT_AT_IP
>> +int rseq_abort_at_ip(struct pt_regs *regs, unsigned long ip);
>> +#endif
>> +
>> #endif /* !__ASSEMBLY__ */
>> #endif /* _ASM_X86_PTRACE_H */
>> diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
>> index 6d2244c94799..561ed98d12ba 100644
>> --- a/arch/x86/kernel/ptrace.c
>> +++ b/arch/x86/kernel/ptrace.c
>> @@ -1368,3 +1368,15 @@ void user_single_step_report(struct pt_regs *regs)
>> {
>> send_sigtrap(regs, 0, TRAP_BRKPT);
>> }
>> +
>> +int rseq_abort_at_ip(struct pt_regs *regs, unsigned long ip)
>> +{
>> + if (user_64bit_mode(regs)) {
>> + /* Need to skip redzone for leaf functions. */
>> + regs->sp -= sizeof(u64) + 128;
>> + return put_user(ip, (u64 __user *)regs->sp);
>> + } else {
>> + regs->sp -= sizeof(u32);
>> + return put_user(ip, (u32 __user *)regs->sp);
>> + }
>
> I think it would be really helpful if you added the full explanation for
> sizeof(u64) + 128 or -sizeof(u32) into this codepath or provide
> constants. For folks not familiar with stuff like this it'll look like
> magic numbers. :)

Good point, here is the planned update:

int rseq_abort_at_ip(struct pt_regs *regs, unsigned long ip)
{
if (user_64bit_mode(regs)) {
/*
* rseq abort-at-ip x86-64 ABI: stack pointer is decremented to
* skip the redzone (128 bytes on x86-64), and decremented of
* the pointer size (8 bytes). The aborted address (abort-at-ip)
* is stored at this updated stack pointer location (top of stack).
*
* Skipping the redzone is needed to make sure not to corrupt
* stack data when the rseq critical section is within a leaf
* function.
*/
regs->sp -= sizeof(u64) + 128;
return put_user(ip, (u64 __user *)regs->sp);
} else {
/*
* rseq abort-at-ip x86-32 ABI: stack pointer is decremented of
* the pointer size (4 bytes). The aborted address (abort-at-ip)
* is stored at this updated stack pointer location (top of stack).
*/
regs->sp -= sizeof(u32);
return put_user(ip, (u32 __user *)regs->sp);
}
}

>
>> +}
>> diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
>> index 9a402fdb60e9..3130232e6d0c 100644
>> --- a/include/uapi/linux/rseq.h
>> +++ b/include/uapi/linux/rseq.h
>> @@ -20,12 +20,14 @@ enum rseq_cpu_id_state {
>>
>> enum rseq_flags {
>> RSEQ_FLAG_UNREGISTER = (1 << 0),
>> + RSEQ_FLAG_QUERY_ABORT_AT_IP = (1 << 1),
>> };
>>
>> enum rseq_cs_flags_bit {
>> RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT_BIT = 0,
>> RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL_BIT = 1,
>> RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE_BIT = 2,
>> + RSEQ_CS_FLAG_ABORT_AT_IP_BIT = 3,
>> };
>>
>> enum rseq_cs_flags {
>> @@ -35,6 +37,8 @@ enum rseq_cs_flags {
>> (1U << RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL_BIT),
>> RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE =
>> (1U << RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE_BIT),
>> + RSEQ_CS_FLAG_ABORT_AT_IP =
>> + (1U << RSEQ_CS_FLAG_ABORT_AT_IP_BIT),
>> };
>>
>> /*
>> diff --git a/kernel/rseq.c b/kernel/rseq.c
>> index 6d45ac3dae7f..fb52f2d11b56 100644
>> --- a/kernel/rseq.c
>> +++ b/kernel/rseq.c
>> @@ -21,6 +21,13 @@
>> #define RSEQ_CS_PREEMPT_MIGRATE_FLAGS (RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE | \
>> RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT)
>>
>> +#ifdef RSEQ_ARCH_HAS_ABORT_AT_IP
>> +static bool rseq_has_abort_at_ip(void) { return true; }
>> +#else
>> +static bool rseq_has_abort_at_ip(void) { return false; }
>> +static int rseq_abort_at_ip(struct pt_regs *regs, unsigned long ip) { return 0;
>> }
>> +#endif
>> +
>> /*
>> *
>> * Restartable sequences are a lightweight interface that allows
>> @@ -79,6 +86,16 @@
>> *
>> * [abort_ip]
>> * F1. <failure>
>> + *
>> + * rseq critical sections defined with the RSEQ_CS_FLAG_ABORT_AT_IP flag
>> + * have the following behavior on abort: when the stack grows down: the
>> + * stack pointer is decremented to skip the redzone, and decremented of
>> + * the pointer size. The aborted address (abort-at-ip) is stored at
>> + * this stack pointer location. The user-space abort handler needs to
>> + * pop the abort-at-ip address from the stack, and add the redzone size
>> + * to the stack pointer.
>> + *
>> + * TODO: describe stack grows up.
>
> Is this intentional or did you forget? :)

Since I did not implement abort-at-ip on stack-grows-up architectures, I felt
it would be too early to describe the algorithm. I can simply remove the TODO
altogether and we'll take care of it when we get there ? If I had to try to
wordsmith it, it would look like e.g.:

* [...] When the stack grows up: the
* stack pointer is incremented to skip the redzone, and incremented of
* the pointer size. The aborted address (abort-at-ip) is stored immediately
* under this stack pointer location. The user-space abort handler needs to
* pop the abort-at-ip address from the stack, and subtract the redzone size
* from the stack pointer.

[ Please let me know if I got somehow confused in my understanding of stack grows
up architectures. ]

I'm also unsure whether any of the stack grows up architecture have redzones ?
From a quick grep for redzone in Linux arch/, only openrisc, powerpc64 and
x86-64 appear to have redzones.

Thanks,

Mathieu

--
Mathieu Desnoyers
EfficiOS Inc.