Dealing with missing system call or ioctl wrappers in Valgrind ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ You're probably reading this because Valgrind bombed out whilst running your program, and advised you to read this file. The good news is that, in general, it's easy to write the missing syscall or ioctl wrappers you need, so that you can continue your debugging. If you send the resulting patches to me, then you'll be doing a favour to all future Valgrind users too. Note that an "ioctl" is just a special kind of system call, really; so there's not a lot of need to distinguish them (at least conceptually) in the discussion that follows. All this machinery is in coregrind/m_syswrap. What are syscall/ioctl wrappers? What do they do? ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Valgrind does what it does, in part, by keeping track of everything your program does. When a system call happens, for example a request to read part of a file, control passes to the Linux kernel, which fulfills the request, and returns control to your program. The problem is that the kernel will often change the status of some part of your program's memory as a result, and tools (instrumentation plug-ins) may need to know about this. Syscall and ioctl wrappers have two jobs: 1. Tell a tool what's about to happen, before the syscall takes place. A tool could perform checks beforehand, eg. if memory about to be written is actually writeable. This part is useful, but not strictly essential. 2. Tell a tool what just happened, after a syscall takes place. This is so it can update its view of the program's state, eg. that memory has just been written to. This step is essential. The "happenings" mostly involve reading/writing of memory. So, let's look at an example of a wrapper for a system call which should be familiar to many Unix programmers. The syscall wrapper for time() ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The wrapper for the time system call looks like this: PRE(sys_time) { /* time_t time(time_t *t); */ PRINT("sys_time ( %p )",ARG1); PRE_REG_READ1(long, "time", int *, t); if (ARG1 != 0) { PRE_MEM_WRITE( "time(t)", ARG1, sizeof(vki_time_t) ); } } POST(sys_time) { if (ARG1 != 0) { POST_MEM_WRITE( ARG1, sizeof(vki_time_t) ); } } The first thing we do happens before the syscall occurs, in the PRE() function. The PRE() function typically starts with invoking to the PRINT() macro. This PRINT() macro implements support for the --trace-syscalls command line option. Next, the tool is told the return type of the syscall, that the syscall has one argument, the type of the syscall argument and that the argument is being read from a register: PRE_REG_READ1(long, "time", int *, t); Next, if a non-NULL buffer is passed in as the argument, tell the tool that the buffer is about to be written to: if (ARG1 != 0) { PRE_MEM_WRITE( "time", ARG1, sizeof(vki_time_t) ); } Finally, the really important bit, after the syscall occurs, in the POST() function: if, and only if, the system call was successful, tell the tool that the memory was written: if (ARG1 != 0) { POST_MEM_WRITE( ARG1, sizeof(vki_time_t) ); } The POST() function won't be called if the syscall failed, so you don't need to worry about checking that in the POST() function. (Note: this is sometimes a bug; some syscalls do return results when they "fail" - for example, nanosleep returns the amount of unslept time if interrupted. TODO: add another per-syscall flag for this case.) Note that we use the type 'vki_time_t'. This is a copy of the kernel type, with 'vki_' prefixed. Our copies of such types are kept in the appropriate vki*.h file(s). We don't include kernel headers or glibc headers directly. Writing your own syscall wrappers (see below for ioctl wrappers) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If Valgrind tells you that system call NNN is unimplemented, do the following: 1. Find out the name of the system call: grep NNN /usr/include/asm/unistd.h This should tell you something like __NR_mysyscallname. Copy this entry to include/vki/vki-scnums-$(VG_PLATFORM).h. 2. Do 'man 2 mysyscallname' to get some idea of what the syscall does. Note that the actual kernel interface can differ from this, so you might also want to check a version of the Linux kernel source. NOTE: any syscall which has something to do with signals or threads is probably "special", and needs more careful handling. Post something to valgrind-developers if you aren't sure. 3. Add a case to the already-huge collection of wrappers in the coregrind/m_syswrap/syswrap-*.c files. For each in-memory parameter which is read or written by the syscall, do one of PRE_MEM_READ( ... ) PRE_MEM_RASCIIZ( ... ) PRE_MEM_WRITE( ... ) for that parameter. Then do the syscall. Then, if the syscall succeeds, issue suitable POST_MEM_WRITE( ... ) calls. (There's no need for POST_MEM_READ calls.) Also, add it to the syscall_table[] array; use one of GENX_, GENXY LINX_, LINXY, PLAX_, PLAXY. GEN* for generic syscalls (in syswrap-generic.c), LIN* for linux specific ones (in syswrap-linux.c) and PLA* for the platform dependant ones (in syswrap-$(PLATFORM)-linux.c). The *XY variant if it requires a PRE() and POST() function, and the *X_ variant if it only requires a PRE() function. If you find this difficult, read the wrappers for other syscalls for ideas. A good tip is to look for the wrapper for a syscall which has a similar behaviour to yours, and use it as a starting point. If you need structure definitions and/or constants for your syscall, copy them from the kernel headers into include/vki.h and co., with the appropriate vki_*/VKI_* name mangling. Don't #include any kernel headers. And certainly don't #include any glibc headers. Test it. Note that a common error is to call POST_MEM_WRITE( ... ) with 0 (NULL) as the first (address) argument. This usually means your logic is slightly inadequate. It's a sufficiently common bug that there's a built-in check for it, and you'll get a "probably sanity check failure" for the syscall wrapper you just made, if this is the case. 4. Once happy, send us the patch. Pretty please. Writing your own ioctl wrappers ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Is pretty much the same as writing syscall wrappers, except that all the action happens within PRE(ioctl) and POST(ioctl). There's a default case, sometimes it isn't correct and you have to write a more specific case to get the right behaviour. As above, please create a bug report and attach the patch as described on http://www.valgrind.org.