Re: [PATCH bpf 5/6] tools: bpftool: resolve calls without using imm field

Hi Jakub,

On 05/18/2018 12:21 AM, Jakub Kicinski wrote:

On Thu, 17 May 2018 12:05:47 +0530, Sandipan Das wrote:

quoted

Currently, we resolve the callee's address for a JITed function
call by using the imm field of the call instruction as an offset
from __bpf_call_base. If bpf_jit_kallsyms is enabled, we further
use this address to get the callee's kernel symbol's name.

For some architectures, such as powerpc64, the imm field is not
large enough to hold this offset. So, instead of assigning this
offset to the imm field, the verifier now assigns the subprog
id. Also, a list of kernel symbol addresses for all the JITed
functions is provided in the program info. We now use the imm
field as an index for this list to lookup a callee's symbol's
address and resolve its name.

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Sandipan Das <redacted>

A few nit-picks below, thank you for the patch!

quoted

 tools/bpf/bpftool/prog.c          | 31 +++++++++++++++++++++++++++++++
 tools/bpf/bpftool/xlated_dumper.c | 24 +++++++++++++++++-------
 tools/bpf/bpftool/xlated_dumper.h |  2 ++
 3 files changed, 50 insertions(+), 7 deletions(-)

diff --git a/tools/bpf/bpftool/prog.c b/tools/bpf/bpftool/prog.c
index 9bdfdf2d3fbe..ac2f62a97e84 100644
--- a/tools/bpf/bpftool/prog.c
+++ b/tools/bpf/bpftool/prog.c

@@ -430,6 +430,10 @@ static int do_dump(int argc, char **argv)
 	unsigned char *buf;
 	__u32 *member_len;
 	__u64 *member_ptr;
+	unsigned int nr_addrs;
+	unsigned long *addrs = NULL;
+	__u32 *ksyms_len;
+	__u64 *ksyms_ptr;

nit: please try to keep the variables ordered longest to shortest like
we do in networking code (please do it in all functions).

quoted

 	ssize_t n;
 	int err;
 	int fd;

@@ -437,6 +441,8 @@ static int do_dump(int argc, char **argv)
 	if (is_prefix(*argv, "jited")) {
 		member_len = &info.jited_prog_len;
 		member_ptr = &info.jited_prog_insns;
+		ksyms_len = &info.nr_jited_ksyms;
+		ksyms_ptr = &info.jited_ksyms;
 	} else if (is_prefix(*argv, "xlated")) {
 		member_len = &info.xlated_prog_len;
 		member_ptr = &info.xlated_prog_insns;

@@ -496,10 +502,23 @@ static int do_dump(int argc, char **argv)
 		return -1;
 	}
 
+	nr_addrs = *ksyms_len;

Here and ...

quoted

+	if (nr_addrs) {
+		addrs = malloc(nr_addrs * sizeof(__u64));
+		if (!addrs) {
+			p_err("mem alloc failed");
+			free(buf);
+			close(fd);
+			return -1;

You can just jump to err_free here.

quoted

+		}
+	}
+
 	memset(&info, 0, sizeof(info));
 
 	*member_ptr = ptr_to_u64(buf);
 	*member_len = buf_size;
+	*ksyms_ptr = ptr_to_u64(addrs);
+	*ksyms_len = nr_addrs;

... here - this function is getting long, so maybe I'm not seeing
something, but are ksyms_ptr and ksyms_len guaranteed to be initialized?

quoted

 	err = bpf_obj_get_info_by_fd(fd, &info, &len);
 	close(fd);

@@ -513,6 +532,11 @@ static int do_dump(int argc, char **argv)
 		goto err_free;
 	}
 
+	if (*ksyms_len > nr_addrs) {
+		p_err("too many addresses returned");
+		goto err_free;
+	}
+
 	if ((member_len == &info.jited_prog_len &&
 	     info.jited_prog_insns == 0) ||
 	    (member_len == &info.xlated_prog_len &&

@@ -558,6 +582,9 @@ static int do_dump(int argc, char **argv)
 			dump_xlated_cfg(buf, *member_len);
 	} else {
 		kernel_syms_load(&dd);
+		dd.jited_ksyms = ksyms_ptr;
+		dd.nr_jited_ksyms = *ksyms_len;
+
 		if (json_output)
 			dump_xlated_json(&dd, buf, *member_len, opcodes);
 		else

@@ -566,10 +593,14 @@ static int do_dump(int argc, char **argv)
 	}
 
 	free(buf);
+	if (addrs)
+		free(addrs);

Free can deal with NULL pointers, no need for an if.

quoted

 	return 0;
 
 err_free:
 	free(buf);
+	if (addrs)
+		free(addrs);
 	return -1;
 }
 

diff --git a/tools/bpf/bpftool/xlated_dumper.c b/tools/bpf/bpftool/xlated_dumper.c
index 7a3173b76c16..dc8e4eca0387 100644
--- a/tools/bpf/bpftool/xlated_dumper.c
+++ b/tools/bpf/bpftool/xlated_dumper.c

@@ -178,8 +178,12 @@ static const char *print_call_pcrel(struct dump_data *dd,
 		snprintf(dd->scratch_buff, sizeof(dd->scratch_buff),
 			 "%+d#%s", insn->off, sym->name);
 	else

else if (address)

saves us the indentation.

quoted

-		snprintf(dd->scratch_buff, sizeof(dd->scratch_buff),
-			 "%+d#0x%lx", insn->off, address);
+		if (address)
+			snprintf(dd->scratch_buff, sizeof(dd->scratch_buff),
+				 "%+d#0x%lx", insn->off, address);
+		else
+			snprintf(dd->scratch_buff, sizeof(dd->scratch_buff),
+				 "%+d", insn->off);
 	return dd->scratch_buff;
 }
 

@@ -200,14 +204,20 @@ static const char *print_call(void *private_data,
 			      const struct bpf_insn *insn)
 {
 	struct dump_data *dd = private_data;
-	unsigned long address = dd->address_call_base + insn->imm;
-	struct kernel_sym *sym;
+	unsigned long address = 0;
+	struct kernel_sym *sym = NULL;
 

Hm.  Quite a bit of churn.  Why not just add these three lines here:

if (insn->src_reg == BPF_PSEUDO_CALL && 
    insn->imm < dd->nr_jited_ksyms)
	address = dd->jited_ksyms[insn->imm];

quoted

-	sym = kernel_syms_search(dd, address);
-	if (insn->src_reg == BPF_PSEUDO_CALL)
+	if (insn->src_reg == BPF_PSEUDO_CALL) {
+		if (dd->nr_jited_ksyms) {
+			address = dd->jited_ksyms[insn->imm];

Perhaps it's paranoid, but it'd please do to bound check insn->imm
against dd->nr_jited_ksyms.

quoted

+			sym = kernel_syms_search(dd, address);
+		}
 		return print_call_pcrel(dd, sym, address, insn);
-	else
+	} else {
+		address = dd->address_call_base + insn->imm;
+		sym = kernel_syms_search(dd, address);
 		return print_call_helper(dd, sym, address);
+	}
 }
 
 static const char *print_imm(void *private_data,

Thank you for the suggestions. Will post out v2 with these changes.

- Sandipan