2a5418a13f
Given we recently had c131187db2d3 ("bpf: fix branch pruning
logic") and 95a762e2c8c9 ("bpf: fix incorrect sign extension in
check_alu_op()") in particular where before verifier skipped
verification of the wrongly assumed dead branch, we should not
just replace the dead code parts with nops (mov r0,r0). If there
is a bug such as fixed in 95a762e2c8c9 in future again, where
runtime could execute those insns, then one of the potential
issues with the current setting would be that given the nops
would be at the end of the program, we could execute out of
bounds at some point.
The best in such case would be to just exit the BPF program
altogether and return an exception code. However, given this
would require two instructions, and such a dead code gap could
just be a single insn long, we would need to place 'r0 = X; ret'
snippet at the very end after the user program or at the start
before the program (where we'd skip that region on prog entry),
and then place unconditional ja's into the dead code gap.
While more complex but possible, there's still another block
in the road that currently prevents from this, namely BPF to
BPF calls. The issue here is that such exception could be
returned from a callee, but the caller would not know that
it's an exception that needs to be propagated further down.
Alternative that has little complexity is to just use a ja-1
code for now which will trap the execution here instead of
silently doing bad things if we ever get there due to bugs.
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.