| |
| 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/vg_syscalls.c. |
| |
| |
| 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 skins (instrumentation plug-ins) may need to know about |
| this. |
| |
| Syscall and ioctl wrappers have two jobs: |
| |
| 1. Tell a skin what's about to happen, before the syscall takes place. A |
| skin 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 skin 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() |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| Removing the debug printing clutter, it looks like this: |
| |
| case __NR_time: /* syscall 13 */ |
| /* time_t time(time_t *t); */ |
| if (arg1 != (UInt)NULL) { |
| SYSCALL_TRACK( pre_mem_write, tst, "time", arg1, sizeof(time_t) ); |
| } |
| KERNEL_DO_SYSCALL(tid,res); |
| if (!VG_(is_kerror)(res) && arg1 != (UInt)NULL) { |
| VG_TRACK( post_mem_write, arg1, sizeof(time_t) ); |
| } |
| break; |
| |
| The first thing we do is, if a non-NULL buffer is passed in as the argument, |
| tell the skin that the buffer is about to be written to: |
| |
| if (arg1 != (UInt)NULL) { |
| SYSCALL_TRACK( pre_mem_write, tst, "time", arg1, sizeof(time_t) ); |
| } |
| |
| Now Valgrind asks the kernel to actally do the system call, for the thread |
| identified by thread ID "tid", depositing the return value in "res": |
| |
| KERNEL_DO_SYSCALL(tid, res); |
| |
| Finally, the really important bit. If, and only if, the system call |
| was successful, tell the skin that the memory was written: |
| |
| if (!VG_(is_kerror)(res) && arg1 != (UInt)NULL) { |
| VG_TRACK( post_mem_write, arg1, sizeof(time_t) ); |
| } |
| |
| The function VG_(is_kerror) tells you whether or not its argument |
| represents a Linux kernel return error code. Hence the test. |
| |
| |
| 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. |
| |
| |
| 2. Do 'man 2 mysyscallname' to get some idea of what the syscall |
| does. |
| |
| |
| 3. Add a case to the already-huge collection of wrappers in |
| coregrind/vg_syscalls.c. For each in-memory parameter which is |
| read or written by the syscall, do one of |
| |
| SYSCALL_TRACK( pre_mem_read, ... ) |
| SYSCALL_TRACK( pre_mem_read_asciiz, ... ) |
| SYSCALL_TRACK( pre_mem_write, ... ) |
| |
| for that parameter. Then do the syscall. Then, if the syscall |
| succeeds, issue suitable VG_TRACK( post_mem_write, ... ) calls. |
| (There's no need for post_mem_read calls.) |
| |
| 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 have to #include headers for structure definitions, |
| put your #includes into vg_unsafe.h. |
| |
| Test it. |
| |
| Note that a common error is to call VG_TRACK( 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. |
| |
| Note that many syscalls are bracketed by #if defined(__NR_mysyscall) |
| ... #endif, because they exist only in the 2.4 kernel and not |
| the 2.2 kernel. This enables the same piece of code to serve both |
| kernels. Please try and stick to this convention. |
| |
| |
| 4. Once happy, send me the patch. Pretty please. |
| |
| |
| |
| |
| Writing your own ioctl wrappers |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| Is pretty much the same as writing syscall wrappers. |
| |
| 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 do send me the resulting patch. |
| |
| |