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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- 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.
http://www.efficios.com