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

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

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
-- 
2.25.1