Re: uprobes: bug in comm/string output?

[Masami Hiramatsu <mhiramat@kernel.org>]

On Wed, 16 Jan 2019 11:16:07 +0100
Andreas Ziegler <andreas.ziegler@fau.de> wrote:

> 
> I went into this a bit deeper today, and right now it is simply failing 
> to parse the code because there is no FETCH_OP_COMM case in 
> process_fetch_insn() for uprobes so that will return -EILSEQ, leading to 
> a make_data_loc(0, ...) in store_trace_args(). If we just add 
> FETCH_OP_COMM and let val point to current->comm (that's what 
> trace_kprobe.c does), we get an -EFAULT return value from 
> fetch_store_string because strncpy_from_user() checks if the argument is 
> in user space.

Correct. I missed to add OP_COMM support. And uprobe's fetch_store_string
is only for user space strings.

> So I think we might need a special case for that, something like 
> FETCH_OP_ST_COMM_STRING which is only used for FETCH_OP_COMM and copies 
> current->comm over to the dynamic area. The implementation could be 
> similar to the old fetch_comm_string implementation before your rewrite.

Hmm, instead, I would like to add current->comm checker and only allows
to copy that. That would be simpler and enough.

Could you test below patch?


tracing: uprobes: Re-enable $comm support for uprobe events

From: Masami Hiramatsu <mhiramat@kernel.org>

Since commit 533059281ee5 ("tracing: probeevent: Introduce new
argument fetching code") dropped the $comm support from uprobe
events, this re-enable it.

For $comm support, use strncpy() instead of strncpy_from_user()
to copy current task's comm. Because it is in the kernel space,
strncpy_from_user() always fails to copy the comm.
This also use strlen() instead of strlen_user() to measure the
length of the comm.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Reported-by: Andreas Ziegler <andreas.ziegler@fau.de>
---
 kernel/trace/trace_uprobe.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
index e335576b9411..97d134e83e0f 100644
--- a/kernel/trace/trace_uprobe.c
+++ b/kernel/trace/trace_uprobe.c
@@ -156,7 +156,10 @@ fetch_store_string(unsigned long addr, void *dest, void *base)
 	if (unlikely(!maxlen))
 		return -ENOMEM;
 
-	ret = strncpy_from_user(dst, src, maxlen);
+	if (addr == (unsigned long)current->comm)
+		ret = strlcpy(dst, current->comm, maxlen);
+	else
+		ret = strncpy_from_user(dst, src, maxlen);
 	if (ret >= 0) {
 		if (ret == maxlen)
 			dst[ret - 1] = '\0';
@@ -173,7 +176,10 @@ fetch_store_strlen(unsigned long addr)
 	int len;
 	void __user *vaddr = (void __force __user *) addr;
 
-	len = strnlen_user(vaddr, MAX_STRING_SIZE);
+	if (addr == (unsigned long)current->comm)
+		len = strlen(current->comm);
+	else
+		len = strnlen_user(vaddr, MAX_STRING_SIZE);
 
 	return (len > MAX_STRING_SIZE) ? 0 : len;
 }
@@ -213,6 +219,9 @@ process_fetch_insn(struct fetch_insn *code, struct pt_regs *regs, void *dest,
 	case FETCH_OP_IMM:
 		val = code->immediate;
 		break;
+	case FETCH_OP_COMM:
+		val = (unsigned long)current->comm;
+		break;
 	case FETCH_OP_FOFFS:
 		val = translate_user_vaddr(code->immediate);
 		break;