Re: [PATCH bpf-next v2 3/8] libbpf: Add weak ksym support to gen_loader

On Oct 13, 2021, at 12:33 AM, Kumar Kartikeya Dwivedi [off-list ref] wrote:

This extends existing ksym relocation code to also support relocating
weak ksyms. Care needs to be taken to zero out the src_reg (currently
BPF_PSEUOD_BTF_ID, always set for gen_loader by bpf_object__relocate_data)
when the BTF ID lookup fails at runtime.  This is not a problem for
libbpf as it only sets ext->is_set when BTF ID lookup succeeds (and only
proceeds in case of failure if ext->is_weak, leading to src_reg
remaining as 0 for weak unresolved ksym).

[...]

	emit(gen, BPF_ALU64_IMM(BPF_RSH, BPF_REG_7, 32));
	emit(gen, BPF_STX_MEM(BPF_W, BPF_REG_8, BPF_REG_7,
			      sizeof(struct bpf_insn) + offsetof(struct bpf_insn, imm)));
-log:
+	emit(gen, BPF_JMP_IMM(BPF_JSGE, BPF_REG_7, 0, 3));
+clear_src_reg:
+	/* clear bpf_object__relocate_data's src_reg assignment, otherwise we get a verifier failure */
+#if defined(__LITTLE_ENDIAN_BITFIELD)
+	reg_mask = 0x0f; /* src_reg,dst_reg,... */
+#elif defined(__BIG_ENDIAN_BITFIELD)
+	reg_mask = 0xf0; /* dst_reg,src_reg,... */
+#else
+#error "Unsupported bit endianness, cannot proceed"
+#endif

nit: Please put the "if defined" block in an inline helper. 

+	emit(gen, BPF_LDX_MEM(BPF_B, BPF_REG_9, BPF_REG_8, offsetofend(struct bpf_insn, code)));
+	emit(gen, BPF_ALU32_IMM(BPF_AND, BPF_REG_9, reg_mask));
+	emit(gen, BPF_STX_MEM(BPF_B, BPF_REG_8, BPF_REG_9, offsetofend(struct bpf_insn, code)));
+
	if (!gen->log_level)
		return;
	emit(gen, BPF_LDX_MEM(BPF_W, BPF_REG_7, BPF_REG_8,