matchBinaries Prefix operator fails to match path longer than 256 chars

[mtardy]

The matchBinaries Prefix operator fails to match a path longer than 256 chars because of the way we read the binary path. We use the exe of the process and walk the dentry from end to beginning. Thus if the path is too long, the buffer contains an incorrect start.

A good situation looks like this:

 <--        buflen         -->
 -----------------------------
 |                /etc/passwd|
 -----------------------------
                 ^
                 |
                buf

A bad situation looks like this:

 <- buflen ->
 -----------
 |tc/passwd|
 -----------
 error = 1

A permanent solution would be to increase the length prepend_name can work on. On a first attempt, it does not seem to be an issue. However, what is blocking is that memcpy and memset will fail on the larger struct (above 1K) even though we make sure those struct are not allocated on the stack (which is 512K).

[mtardy]

For context: cc @anfedotoff and related to #2718.

[mtardy]

yay, I have a PoC, I'll prepare a patch and send a PR for that. :)