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

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