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Levin" Cc: Andrea Bolognani , WANG Xuerui , Jiaxun Yang , Huacai Chen , Felix Yan , Ruizhe Pan , Shiqi Zhang , Guo Ren , Yao Zi , Yangyu Chen , Han Gao , linux-kernel@vger.kernel.org, rsworktech@outlook.com References: <59505464-c84a-403d-972f-d4b2055eeaac@gmail.com> <6b2ff48a-ab43-4866-af5a-b8b7d3c23582@ghiti.fr> Content-Language: en-GB-large In-Reply-To: <6b2ff48a-ab43-4866-af5a-b8b7d3c23582@ghiti.fr> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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. > > 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/ >> >> >> _______________________________________________ >> linux-riscv mailing list >> linux-riscv@lists.infradead.org >> http://lists.infradead.org/mailman/listinfo/linux-riscv