alt Hacker Newssplice is documented to return EBADF if "One or both file descriptors are not valid, or do not have proper read-write mode."
So it seems surprising to me that you can call it when the out fd is not writable? But I didn't retain the information about the vulnerability, so I'm missing something. There was something about copy on write, IIRC?
"proper read-write mode" for the input fd is reading only. The exploit is writing to the splice() input fd.
Also, NB, I said permission check, not mode check. The input fd to splice can and will be open for only reading quite often. Doesn't mean the kernel can't still do a write permission check.
(Except I didn't say that here. Oops. Getting confused with my posts.)