On 3/11/16 2:24 AM, Daniel Borkmann wrote:
On 03/10/2016 05:02 AM, Alexei Starovoitov wrote:Lots of places in the kernel use memcpy(buf, comm, TASK_COMM_LEN); but the result is typically passed to print("%s", buf) and extra bytes after zero don't cause any harm. In bpf the result of bpf_get_current_comm() is used as the part of map key and was causing spurious hash map mismatches. Use strlcpy() to guarantee zero-terminated string. bpf verifier checks that output buffer is zero-initialized,Sorry for late reply, more below:so even for short task names the output buffer don't have junk bytes. Note it's not a security concern, since kprobe+bpf is root only. Fixes: ffeedafbf023 ("bpf: introduce current->pid, tgid, uid, gid, comm accessors") Reported-by: Tobias Waldekranz <[email protected]> Signed-off-by: Alexei Starovoitov <[email protected]>[...]diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c index 4504ca66118d..50da680c479f 100644 --- a/kernel/bpf/helpers.c +++ b/kernel/bpf/helpers.c @@ -166,7 +166,7 @@ static u64 bpf_get_current_comm(u64 r1, u64 size, u64 r3, u64 r4, u64 r5) if (!task) return -EINVAL; - memcpy(buf, task->comm, min_t(size_t, size, sizeof(task->comm))); + strlcpy(buf, task->comm, min_t(size_t, size, sizeof(task->comm)));If I see this correctly, __set_task_comm() makes sure comm is always zero terminated, so that seems good, but isn't it already sufficient when switching to strlcpy() to simply use: strlcpy(buf, task->comm, size); The min_t() seems unnecessary work to me, why do we still need it? size is guaranteed to be > 0 through the eBPF verifier, so strlcpy() should take care of the rest.
that's one clever optimization. yep. we can drop min_t. btw I wanted to add memset to __set_task_comm, keep memcpy in bpf_get_current_comm and optimize perf_event_comm_event (which doing: memset+strlcpy and can be replaced with memcpy), but figured that such 'fix' is not suitable for stable. I guess we can do in the next cycle? strlen is not cheap. Especially since it turned out that bpf_get_current_comm() is used very often in the hot path in bcc/tools. Also for the next cycle I'm planning to extend verifier to allow uninitialized stack to be passed to functions like bpf_get_current_comm() and they would have to zero it in error cases. Then we can save few more cycles from the programs.
