[PATCH bpf v2 3/6] bpf: get kernel symbol addresses via syscall

[Sandipan Das]

This adds new two new fields to struct bpf_prog_info. For
multi-function programs, these fields can be used to pass
a list of kernel symbol addresses for all functions in a
given program and to userspace using the bpf system call
with the BPF_OBJ_GET_INFO_BY_FD command.

When bpf_jit_kallsyms is enabled, we can get the address
of the corresponding kernel symbol for a callee function
and resolve the symbol's name. The address is determined
by adding the value of the call instruction's imm field
to __bpf_call_base. This offset gets assigned to the imm
field by the verifier.

For some architectures, such as powerpc64, the imm field
is not large enough to hold this offset.

We resolve this by:

[1] Assigning the subprog id to the imm field of a call
    instruction in the verifier instead of the offset of
    the callee's symbol's address from __bpf_call_base.

[2] Determining the address of a callee's corresponding
    symbol by using the imm field as an index for the
    list of kernel symbol addresses now available from
    the program info.

Suggested-by: Daniel Borkmann <daniel at iogearbox.net>
Signed-off-by: Sandipan Das <sandipan at linux.vnet.ibm.com>
---
 include/uapi/linux/bpf.h |  2 ++
 kernel/bpf/syscall.c     | 20 ++++++++++++++++++++
 kernel/bpf/verifier.c    |  7 +------
 3 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index d94d333a8225..040c9cac7303 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -2188,6 +2188,8 @@ struct bpf_prog_info {
 	__u32 xlated_prog_len;
 	__aligned_u64 jited_prog_insns;
 	__aligned_u64 xlated_prog_insns;
+	__aligned_u64 jited_ksyms;
+	__u32 nr_jited_ksyms;
 	__u64 load_time;	/* ns since boottime */
 	__u32 created_by_uid;
 	__u32 nr_map_ids;
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index bfcde949c7f8..54a72fafe57c 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -1933,6 +1933,7 @@ static int bpf_prog_get_info_by_fd(struct bpf_prog *prog,
 	if (!capable(CAP_SYS_ADMIN)) {
 		info.jited_prog_len = 0;
 		info.xlated_prog_len = 0;
+		info.nr_jited_ksyms = 0;
 		goto done;
 	}
 
@@ -1981,6 +1982,25 @@ static int bpf_prog_get_info_by_fd(struct bpf_prog *prog,
 		}
 	}
 
+	ulen = info.nr_jited_ksyms;
+	info.nr_jited_ksyms = prog->aux->func_cnt;
+	if (info.nr_jited_ksyms && ulen) {
+		u64 __user *user_jited_ksyms = u64_to_user_ptr(info.jited_ksyms);
+		ulong ksym_addr;
+		u32 i;
+
+		/* copy the address of the kernel symbol corresponding to
+		 * each function
+		 */
+		ulen = min_t(u32, info.nr_jited_ksyms, ulen);
+		for (i = 0; i < ulen; i++) {
+			ksym_addr = (ulong) prog->aux->func[i]->bpf_func;
+			ksym_addr &= PAGE_MASK;
+			if (put_user((u64) ksym_addr, &user_jited_ksyms[i]))
+				return -EFAULT;
+		}
+	}
+
 done:
 	if (copy_to_user(uinfo, &info, info_len) ||
 	    put_user(info_len, &uattr->info.info_len))
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 6c56cce9c4e3..e826c396aba2 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -5426,17 +5426,12 @@ static int jit_subprogs(struct bpf_verifier_env *env)
 	 * later look the same as if they were interpreted only.
 	 */
 	for (i = 0, insn = prog->insnsi; i < prog->len; i++, insn++) {
-		unsigned long addr;
-
 		if (insn->code != (BPF_JMP | BPF_CALL) ||
 		    insn->src_reg != BPF_PSEUDO_CALL)
 			continue;
 		insn->off = env->insn_aux_data[i].call_imm;
 		subprog = find_subprog(env, i + insn->off + 1);
-		addr  = (unsigned long)func[subprog]->bpf_func;
-		addr &= PAGE_MASK;
-		insn->imm = (u64 (*)(u64, u64, u64, u64, u64))
-			    addr - __bpf_call_base;
+		insn->imm = subprog;
 	}
 
 	prog->jited = 1;
-- 
2.14.3