Document the role of PTRACE_O_TRACESYSGOOD option in connection with PTRACE_GET_SYSCALL_INFO. Came upon this after writing a test program using PTRACE_GET_SYSCALL_INFO. After failing to find what's wrong I posted a StackOverflow question which you can find right here: <https://stackoverflow.com/questions/72410182/ptrace-get-syscall-info-always-returns-info-op-as-ptrace-syscall-info-none> Nate Eldredge found out what happens by looking into the kernel's source code, here is a link to the relevant part <https://github.com/torvalds/linux/blob/8291eaafed36f575f23951f3ce18407f480e9ecf/kernel/ptrace.c#L1018> In the code it can be seen that in case of system call entry or exit stops, the union is filled if and only if the signal matches "SIGTRAP | 0x80", a signal which is only sent if the PTRACE_O_TRACESYSGOOD option is set. You can read about that in the PTRACE_O_TRACESYSGOOD section of ptrace(2)'s manual. Complements: fc91449cb "ptrace.2: Document PTRACE_GET_SYSCALL_INFO" Cowritten-by: Dmitry V. Levin <ldv@xxxxxxxxx> Signed-off-by: Dmitry V. Levin <ldv@xxxxxxxxx> Signed-off-by: Fotios Valasiadis <fvalasiad@xxxxxxxxx> Acked-by: Nate Eldredge <nate@xxxxxxxxxxxxxxxxxxxx> Cc: Elvira Khabirova <lineprinter0@xxxxxxxxx> Signed-off-by: Alejandro Colomar <alx@xxxxxxxxxx> --- man2/ptrace.2 | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/man2/ptrace.2 b/man2/ptrace.2 index 55d9fd36d..9737b3825 100644 --- a/man2/ptrace.2 +++ b/man2/ptrace.2 @@ -1111,6 +1111,15 @@ stop. .B PTRACE_SYSCALL_INFO_NONE No component of the union contains relevant information. .RE +.IP +In case of system call entry or exit stops, +the data returned by +.B PTRACE_GET_SYSCALL_INFO +is limited to type +.B PTRACE_SYSCALL_INFO_NONE +unless +.B PTRACE_O_TRACESYSGOOD +option is set before the corresponding system call stop has occurred. .\" .SS Death under ptrace When a (possibly multithreaded) process receives a killing signal -- 2.39.2
