Clang patch [0] enables emitting relocatable generic ALU/ALU64 instructions (i.e, shifts and arithmetic operations), as well as generic load/store instructions. The former ones are already supported by libbpf as is. This patch adds further support for load/store instructions. Relocatable field offset is encoded in BPF instruction's 16-bit offset section and are adjusted by libbpf based on target kernel BTF. These Clang changes and corresponding libbpf changes allow for more succinct generated BPF code by encoding relocatable field reads as a single ST/LDX/STX instruction. It also enables relocatable access to BPF context. Previously, if context struct (e.g., __sk_buff) was accessed with CO-RE relocations (e.g., due to preserve_access_index attribute), it would be rejected by BPF verifier due to modified context pointer dereference. With Clang patch, such context accesses are both relocatable and have a fixed offset from the point of view of BPF verifier. [0] https://reviews.llvm.org/D71790 Signed-off-by: Andrii Nakryiko <andriin@xxxxxx> --- tools/lib/bpf/libbpf.c | 31 ++++++++++++++++++++++++++++--- 1 file changed, 28 insertions(+), 3 deletions(-) diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c index 9576a90c5a1c..7513165b104f 100644 --- a/tools/lib/bpf/libbpf.c +++ b/tools/lib/bpf/libbpf.c @@ -18,6 +18,7 @@ #include <stdarg.h> #include <libgen.h> #include <inttypes.h> +#include <limits.h> #include <string.h> #include <unistd.h> #include <endian.h> @@ -3810,11 +3811,13 @@ static int bpf_core_reloc_insn(struct bpf_program *prog, insn = &prog->insns[insn_idx]; class = BPF_CLASS(insn->code); - if (class == BPF_ALU || class == BPF_ALU64) { + switch (class) { + case BPF_ALU: + case BPF_ALU64: if (BPF_SRC(insn->code) != BPF_K) return -EINVAL; if (!failed && validate && insn->imm != orig_val) { - pr_warn("prog '%s': unexpected insn #%d value: got %u, exp %u -> %u\n", + pr_warn("prog '%s': unexpected insn #%d (ALU/ALU64) value: got %u, exp %u -> %u\n", bpf_program__title(prog, false), insn_idx, insn->imm, orig_val, new_val); return -EINVAL; @@ -3824,7 +3827,29 @@ static int bpf_core_reloc_insn(struct bpf_program *prog, pr_debug("prog '%s': patched insn #%d (ALU/ALU64)%s imm %u -> %u\n", bpf_program__title(prog, false), insn_idx, failed ? " w/ failed reloc" : "", orig_val, new_val); - } else { + break; + case BPF_LDX: + case BPF_ST: + case BPF_STX: + if (!failed && validate && insn->off != orig_val) { + pr_warn("prog '%s': unexpected insn #%d (LD/LDX/ST/STX) value: got %u, exp %u -> %u\n", + bpf_program__title(prog, false), insn_idx, + insn->off, orig_val, new_val); + return -EINVAL; + } + if (new_val > SHRT_MAX) { + pr_warn("prog '%s': insn #%d (LD/LDX/ST/STX) value too big: %u\n", + bpf_program__title(prog, false), insn_idx, + new_val); + return -ERANGE; + } + orig_val = insn->off; + insn->off = new_val; + pr_debug("prog '%s': patched insn #%d (LD/LDX/ST/STX)%s off %u -> %u\n", + bpf_program__title(prog, false), insn_idx, + failed ? " w/ failed reloc" : "", orig_val, new_val); + break; + default: pr_warn("prog '%s': trying to relocate unrecognized insn #%d, code:%x, src:%x, dst:%x, off:%x, imm:%x\n", bpf_program__title(prog, false), insn_idx, insn->code, insn->src_reg, insn->dst_reg, -- 2.17.1
