On Fri, Feb 24, 2023 at 09:11:55PM +0200, Φώτης Βαλασιάδης wrote: > This was during the early days of the program I was implementing, I > later found out I needed PTRACE_O_TRACESYSGOOD anyways as the manual > suggests. Still it's something that should be documented! Caused a lot > of frustration. > > Regarding your suggestion, it's great but I have something to point out. > In the case of PTRACE_EVENT_SECCOMP the union is filled normally, > without any need for the PTRACE_O_TRACESYSGOOD option to be set. So > unless I am missing something, please consider including that! Correct, to get PTRACE_SYSCALL_INFO_SECCOMP one has to set PTRACE_O_TRACESECCOMP option, and to get PTRACE_SYSCALL_INFO_ENTRY/PTRACE_SYSCALL_INFO_EXIT one has to set PTRACE_O_TRACESYSGOOD option. > On 24/2/23 20:53, Dmitry V. Levin wrote: > > 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
