bpf: add bpf_patch_insn_single helper

Move the functionality to patch instructions out of the verifier
code and into the core as the new bpf_patch_insn_single() helper
will be needed later on for blinding as well. No changes in
functionality.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 71 insertions, 0 deletions

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 5313d09..49b5538 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -136,6 +136,77 @@ void __bpf_prog_free(struct bpf_prog *fp)
 	vfree(fp);
 }
 
+static bool bpf_is_jmp_and_has_target(const struct bpf_insn *insn)
+{
+	return BPF_CLASS(insn->code) == BPF_JMP  &&
+	       /* Call and Exit are both special jumps with no
+		* target inside the BPF instruction image.
+		*/
+	       BPF_OP(insn->code) != BPF_CALL &&
+	       BPF_OP(insn->code) != BPF_EXIT;
+}
+
+static void bpf_adj_branches(struct bpf_prog *prog, u32 pos, u32 delta)
+{
+	struct bpf_insn *insn = prog->insnsi;
+	u32 i, insn_cnt = prog->len;
+
+	for (i = 0; i < insn_cnt; i++, insn++) {
+		if (!bpf_is_jmp_and_has_target(insn))
+			continue;
+
+		/* Adjust offset of jmps if we cross boundaries. */
+		if (i < pos && i + insn->off + 1 > pos)
+			insn->off += delta;
+		else if (i > pos + delta && i + insn->off + 1 <= pos + delta)
+			insn->off -= delta;
+	}
+}
+
+struct bpf_prog *bpf_patch_insn_single(struct bpf_prog *prog, u32 off,
+				       const struct bpf_insn *patch, u32 len)
+{
+	u32 insn_adj_cnt, insn_rest, insn_delta = len - 1;
+	struct bpf_prog *prog_adj;
+
+	/* Since our patchlet doesn't expand the image, we're done. */
+	if (insn_delta == 0) {
+		memcpy(prog->insnsi + off, patch, sizeof(*patch));
+		return prog;
+	}
+
+	insn_adj_cnt = prog->len + insn_delta;
+
+	/* Several new instructions need to be inserted. Make room
+	 * for them. Likely, there's no need for a new allocation as
+	 * last page could have large enough tailroom.
+	 */
+	prog_adj = bpf_prog_realloc(prog, bpf_prog_size(insn_adj_cnt),
+				    GFP_USER);
+	if (!prog_adj)
+		return NULL;
+
+	prog_adj->len = insn_adj_cnt;
+
+	/* Patching happens in 3 steps:
+	 *
+	 * 1) Move over tail of insnsi from next instruction onwards,
+	 *    so we can patch the single target insn with one or more
+	 *    new ones (patching is always from 1 to n insns, n > 0).
+	 * 2) Inject new instructions at the target location.
+	 * 3) Adjust branch offsets if necessary.
+	 */
+	insn_rest = insn_adj_cnt - off - len;
+
+	memmove(prog_adj->insnsi + off + len, prog_adj->insnsi + off + 1,
+		sizeof(*patch) * insn_rest);
+	memcpy(prog_adj->insnsi + off, patch, sizeof(*patch) * len);
+
+	bpf_adj_branches(prog_adj, off, insn_delta);
+
+	return prog_adj;
+}
+
 #ifdef CONFIG_BPF_JIT
 struct bpf_binary_header *
 bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,