On Fri, Feb 24, 2023 at 06:31:42PM +0200, Fotios Valasiadis wrote: > From: Fotios Valasiadis <fvalasiad@xxxxxxxxxxx> > > Fixed incomplete doc. PTRACE_GET_SYSCALL_INFO's description wouldn't inform the user that they need to enable PTRACE_O_TRACESYSGOOD to get detailed information from said operation. > > Came upon this bug after writing a test program using PTRACE_O_TRACESYSGOOD. 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's wrong 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 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. > > I am unaware if Nate wants to be included as a co-author of this commit, nor am I aware of any way to contact him. Cc'ed Nate just in case. > Signed-off-by: Fotios Valasiadis <fvalasiad@xxxxxxxxx> > --- > man2/ptrace.2 | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/man2/ptrace.2 b/man2/ptrace.2 > index 55d9fd36d..b437b4f44 100644 > --- a/man2/ptrace.2 > +++ b/man2/ptrace.2 > @@ -1033,6 +1033,11 @@ If the size of the data to be written by the kernel exceeds the size > specified by the > .I addr > argument, the output data is truncated. > +To obtain complete information during system call entry or exit stops, > +you should enable the > +.B PTRACE_O_TRACESYSGOOD > +option using > +.B PTRACE_SETOPTIONS. > .IP > The > .I ptrace_syscall_info I suggest the following wording: diff --git a/man2/ptrace.2 b/man2/ptrace.2 index 55d9fd36d..496d8fae0 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 +Note that 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 ptrace stop has occurred. .\" .SS Death under ptrace When a (possibly multithreaded) process receives a killing signal As written in the manual page, using the PTRACE_O_TRACESYSGOOD option is the recommended method to distinguish syscall-stops from other kinds of ptrace-stops, since it is reliable and does not incur a performance penalty. In other words, if you use ptrace(2) at all, please consider using PTRACE_O_TRACESYSGOOD option. -- ldv
