[RFC] riscv/entry: issue about a0/orig_a0 register and ENOSYS

[Celeste Liu]

On 2024-10-16 20:56, Alexandre Ghiti wrote:
> On 16/10/2024 14:23, Celeste Liu wrote:
>> On 2024-10-16 20:00, Alexandre Ghiti wrote:
>>> Hi Celeste,
>>>
>>> Thank you for looking into this and really sorry about the late response.
>>>
>>> On 17/09/2024 06:09, Celeste Liu wrote:
>>>> Before PTRACE_GET_SYSCALL_INFO was implemented in v5.3, the only way to
>>>> get syscall arguments was to get user_regs_struct via PTRACE_GETREGSET.
>>>> On some architectures where a register is used as both the first
>>>> argument and the return value and thus will be changed at some stage of
>>>> the syscall process, something like orig_a0 is provided to save the
>>>> original argument register value. But RISC-V doesn't export orig_a0 in
>>>> user_regs_struct (This ABI was designed at e2c0cdfba7f6 ("RISC-V:
>>>> User-facing API").) so userspace application like strace will get the
>>>> right or wrong result depends on the operation order in do_trap_ecall_u()
>>>> function.
>>>>
>>>> This requires we put the ENOSYS process after syscall_enter_from_user_mode()
>>>> or syscall_handler()[1]. Unfortunately, the generic entry API
>>>> syscall_enter_from_user_mode() requires we
>>>>
>>>> *  process ENOSYS before syscall_enter_from_user_mode()
>>>
>>> Where does this requirement come from?
>>>
>>>
>>>> *  or only set a0 to ENOSYS when the return value of
>>>>      syscall_enter_from_user_mode() != -1
>>>>
>>>> Again, if we choose the latter way to avoid conflict with the first
>>>> issue, we will meet the third problem: strace depends on that kernel
>>>> will return ENOSYS when syscall nr is -1 to implement their syscall
>>>> tampering feature[2].
>>>
>>> IIUC, seccomp and others in syscall_enter_from_user_mode() could return -1 and then we could not differentiate with the syscall number being -1.
>>>
>>> But could we imagine, to distinguish between an error and the syscall number being -1, checking again the syscall number after we call syscall_enter_from_user_mode()? If the syscall number is -1, then we set ENOSYS otherwise we don't do anything (a bit like what you did in 52449c17bdd1 ("riscv: entry: set a0 = -ENOSYS only when syscall != -1")).
>>>
>>> Let me know if I completely misunderstood here!
>> Yeah. I found this a bit later after I post this RFC. I include it in a update reply,
>> copy here as well:
>>
>>> But from another angle, syscall number is in a7 register, so we can call the
>>> get_syscall_nr() after calling the syscall_enter_from_user_mode() to bypass the
>>> information lost of the return value of the syscall_enter_from_user_mode(). But
>>> in this way, the syscall number in the syscall_enter_from_user_mode() return
>>> value is useless, and we can remove it directly.
>> So if we get syscall number from a7 register again, the syscall number part of
>> the return value of syscall_enter_from_user_mode() is useless completely.
>> I think it's better to drop it so the later new architecture developer will not
>> run into the same issue. (Actually, the syscall number returned by
>> syscall_enter_from_user_mode() is also the result of get_syscall_nr() at the end
>> of it.) But it will affect other architecture's code so I think there still need
>> some discussions.
>>
>> Or if you think it's better to post a patch and then discuss in patch thread
>> directly, I'm glad to do this.
> 
> 
> Great that we have a solution that does not need to change the ABI :)
> 
> I think we should start by implementing a fix for riscv only that implements the get_syscall_nr() after syscall_enter_from_user_mode() so that we can merge that in 6.12-rcX.
> 
> And after that, you could come with the nicer solution you propose.
> 
> Do you think you can send a patch for the quick fix soon? In the meantime, I'm adding the strace testsuite to my CI to make sure it works and we don't break it again :)

Ok. I will send patch soon.

> 
> Thanks!
> 
> Alex
> 
> 
>>
>>> Thanks again for the thorough explanation,
>>>
>>> Alex
>>>
>>>
>>>> Actually, we tried the both ways in 52449c17bdd1 ("riscv: entry: set
>>>> a0 = -ENOSYS only when syscall != -1") and 61119394631f ("riscv: entry:
>>>> always initialize regs->a0 to -ENOSYS") before.
>>>>
>>>> Naturally, there is a solution:
>>>>
>>>> 1. Just add orig_a0 in user_regs_struct and let strace use it as
>>>>      loongarch does. So only two problems, which can be resolved without
>>>>      conflict, are left here.
>>>>
>>>> The conflicts are the direct result of the limitation of generic entry
>>>> API, so we have another two solutions:
>>>>
>>>> 2. Give up the generic entry API, and switch back to the
>>>>      architecture-specific standardalone implementation.
>>>> 3. Redesign the generic entry API: the problem was caused by
>>>>      syscall_enter_from_user_mode() using the value -1 (which is unused
>>>>      normally) of syscall nr to inform syscall was reject by seccomp/bpf.
>>>>
>>>> In theory, the Solution 1 is best:
>>>>
>>>> *  a0 was used for two purposes in ABI, so using two variables to store
>>>>      it is natural.
>>>> *  Userspace can implement features without depending on the internal
>>>>      behavior of the kernel.
>>>>
>>>> Unfortunately, it's difficult to implement based on the current code.
>>>> The RISC-V defined struct pt_regs as below:
>>>>
>>>>           struct pt_regs {
>>>>                   unsigned long epc;
>>>>           ...
>>>>                   unsigned long t6;
>>>>                   /* Supervisor/Machine CSRs */
>>>>                   unsigned long status;
>>>>                   unsigned long badaddr;
>>>>                   unsigned long cause;
>>>>                   /* a0 value before the syscall */
>>>>                   unsigned long orig_a0;
>>>>           };
>>>>
>>>> And user_regs_struct needs to be a prefix of struct pt_regs, so if we
>>>> want to include orig_a0 in user_regs_struct, we will need to include
>>>> Supervisor/Machine CSRs as well. It's not a big problem. Since
>>>> struct pt_regs is the internal ABI of the kernel, we can reorder it.
>>>> Unfortunately, struct user_regs_struct is defined as below:
>>>>
>>>>           struct user_regs_struct {
>>>>                   unsigned long pc;
>>>>           ...
>>>>                   unsigned long t6;
>>>>           };
>>>>
>>>> It doesn't contain something like reserved[] as padding to leave the
>>>> space to add more registers from struct pt_regs!
>>>> The loongarch do the right thing as below:
>>>>
>>>>           struct user_pt_regs {
>>>>                   /* Main processor registers. */
>>>>                   unsigned long regs[32];
>>>>           ...
>>>>                   unsigned long reserved[10];
>>>>           } __attribute__((aligned(8)));
>>>>
>>>> RISC-V can't include orig_a0 in user_regs_struct without breaking UABI.
>>>>
>>>> Need a discussion to decide to use which solution, or is there any
>>>> other better solution?
>>>>
>>>> [1]: https://github.com/strace/strace/issues/315
>>>> [2]: https://lore.kernel.org/linux-riscv/20240627071422.GA2626@altlinux.org/
>>>>
>>>>
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