[PATCH] uprobe: Find last occurrence of ':' when parsing uprobe PATH:OFFSET

[Kenny Yu <kennyyu@fb.com>]

Previously, `create_trace_uprobe` found the *first* occurence
of the ':' character when parsing `PATH:OFFSET` for a uprobe.
However, if the path contains a ':' character, then the function
would parse the path incorrectly. Even worse, if the path does not
exist, the subsequent call to `kern_path()` would set `ret` to
`ENOENT`, leading to very cryptic errno values in user space.

The fix is to find the *last* occurence of ':'.

How to repro:: The write fails with "No such file or directory", suggesting
incorrectly that the `uprobe_events` file does not exist.

  $ mkdir testing && cd testing
  $ cp /bin/bash .
  $ cp /bin/bash ./bash:with:colon
  $ echo "p:uprobes/p__root_testing_bash_0x6 /root/testing/bash:0x6" > /sys/kernel/debug/tracing/uprobe_events     # this works
  $ echo "p:uprobes/p__root_testing_bash_with_colon_0x6 /root/testing/bash:with:colon:0x6" >> /sys/kernel/debug/tracing/uprobe_events     # this doesn't
  -bash: echo: write error: No such file or directory

With the patch:

  $ echo "p:uprobes/p__root_testing_bash_0x6 /root/testing/bash:0x6" > /sys/kernel/debug/tracing/uprobe_events     # this still works
  $ echo "p:uprobes/p__root_testing_bash_with_colon_0x6 /root/testing/bash:with:colon:0x6" >> /sys/kernel/debug/tracing/uprobe_events     # this works now too!
  $ cat /sys/kernel/debug/tracing/uprobe_events
  p:uprobes/p__root_testing_bash_0x6 /root/testing/bash:0x0000000000000006
  p:uprobes/p__root_testing_bash_with_colon_0x6 /root/testing/bash:with:colon:0x0000000000000006

Signed-off-by: Kenny Yu <kennyyu@fb.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
---
 kernel/trace/trace_uprobe.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
index c534854..b916b26 100644
--- a/kernel/trace/trace_uprobe.c
+++ b/kernel/trace/trace_uprobe.c
@@ -431,7 +431,8 @@ static int create_trace_uprobe(int argc, char **argv)
 		pr_info("probe point must be have a filename.\n");
 		return -EINVAL;
 	}
-	arg = strchr(argv[1], ':');
+	/* Find the last occurrence, in case the path contains ':' too. */
+	arg = strrchr(argv[1], ':');
 	if (!arg) {
 		ret = -EINVAL;
 		goto fail_address_parse;
-- 
2.9.3