Re: [PATCH v2 0/1] Remove forget_syscall() from start_thread_common()

[Francis Laniel <flaniel@linux.microsoft.com>]

Hi.

Le mercredi 8 juin 2022, 18:24:45 CEST Francis Laniel a écrit :
> Hi.
> 
> 
> First, I hope you are fine and the same for your relatives.
> 
> With this contribution, I enabled using syscalls:sys_exit_execve and
> syscalls:sys_exit_execveat as tracepoints on arm64.
> Indeed, before this contribution, the above tracepoint would not print their
> information as syscall number was set to -1 by calling forget_syscall().
> 
> I tested it by compiling a kernel for arm64 and running it within a VM:
> # Perf was compiled with linux kernel source.
> root@vm-arm64:~# perf record -ag -e 'syscalls:sys_exit_execve' -e
> 'syscalls:sys_enter_execve' & [1] 263
> root@vm-arm64:~# ls
> perf.data  share
> root@vm-arm64:~# fg
> perf record -ag -e 'syscalls:sys_exit_execve' -e 'syscalls:sys_enter_execve'
> ^C[ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.061 MB perf.data (2 samples) ]
> root@vm-arm64:~# perf script
> bash   264 [000]    66.220187: syscalls:sys_enter_execve: filename:
> 0xaaab05d9d ...
> # Below line does not appear without this patch.
> ls   264 [000]    66.226848:  syscalls:sys_exit_execve: 0x0
> ...
> 
> Forgetting the syscall number before starting a new thread was confirmed to
> be a bug [1].
> Particularly, the following architectures do not forget the syscall number
> before starting a new thread:
> * arm (32 bits) EABI: start_thread() sets r7 to previous r7 for ELF FDPIC 
> and to 0 for other binfmts [2].
> * arm (32 bits) OABI: syscall number is set to -1 if
> ptrace_report_syscall_entry() failed [3].
> * mips: start_thread() does not modify current_thread_info->syscall which is
> taken directly from v0 [4, 5].
> * riscv: start_thread() does not modify a7 [6].
> * x86_64: start_thread_common() does not touch orig_ax which seems to
> contain the syscall number [7].
> 
> If you see any way to improve this contribution, feel free to share!
> 
> Change since:
> v1:
>  * Remove call to forget_syscall() and store previous syscall number in
>  regs->syscallno unconditionnaly.
> 
> Francis Laniel (1):
>   arm64: Do not forget syscall when starting a new thread.
> 
>  arch/arm64/include/asm/processor.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> 
> Best regards and thank you in advance.
> ---
> [1] https://lore.kernel.org/linux-arm-kernel/YoT1iLPEbteRTQGZ@arm.com/
> [2] https://elixir.bootlin.com/linux/v5.18-rc6/source/arch/arm/include/asm/
> processor.h#L52
> [3] https://elixir.bootlin.com/linux/v5.18-rc6/source/arch/arm/kernel/
> ptrace.c#L847
> [4] https://elixir.bootlin.com/linux/v5.18-rc6/source/arch/mips/kernel/
> process.c#L52
> [5] https://elixir.bootlin.com/linux/v5.18-rc6/source/arch/mips/kernel/
> scall64-n64.S#L85
> [6] https://elixir.bootlin.com/linux/v5.18-rc6/source/arch/riscv/kernel/
> process.c#L87
> [7] https://elixir.bootlin.com/linux/v5.18-rc6/source/arch/x86/kernel/
> process_64.c#L505

Can someone please take a quick look at this patch?


Best regards and thank you in advance.