[PATCH bpf v2 0/2] Fix partial copy of non-linear test_run output

[PATCH bpf v2 0/2] Fix partial copy of non-linear test_run output

[Sun Jian]

When BPF_PROG_TEST_RUN returns non-linear output and userspace provides a
short data_out buffer, bpf_test_finish() can return -ENOSPC before copying
the packet prefix or updating data_size_out.

Fix this by deriving the linear copy length from the packet layout rather
than from the already-clamped copy_size. Add selftest coverage for both
non-linear skb and XDP frags paths.

---

Changes in v2:

* Fix the Fixes tag to point to the commit that introduced the shared
  non-linear copy-out logic.
* Drop skb-specific wording from the fix commit.
* Move the selftest from skb_load_bytes.c to prog_run_opts.c.
* Add XDP frags coverage in addition to non-linear skb coverage.

v1:
https://lore.kernel.org/bpf/20260615073856.152479-1-sun.jian.kdev@gmail.com/

Tested with:
  ./test_progs -t prog_run_opts -v
  ./test_progs -t skb_load_bytes -v
  ./test_progs -t xdp_pull_data -v

Sun Jian (2):
  bpf: Fix partial copy of non-linear test_run output
  selftests/bpf: Cover partial copy of non-linear test_run output

 net/bpf/test_run.c                            | 11 ++-
 .../selftests/bpf/prog_tests/prog_run_opts.c  | 72 +++++++++++++++++++
 .../selftests/bpf/progs/test_pkt_access.c     | 12 ++++
 3 files changed, 88 insertions(+), 7 deletions(-)

Range-diff:
1:  3691b07aa440 ! 1:  e5a0c426d4cb bpf: Fix partial copy of non-linear skb test_run output
    @@ Metadata
     Author: Sun Jian <sun.jian.kdev@gmail.com>
     
      ## Commit message ##
    -    bpf: Fix partial copy of non-linear skb test_run output
    +    bpf: Fix partial copy of non-linear test_run output
     
    -    For non-linear skbs, bpf_test_finish() derives the linear head copy
    -    length from copy_size - frag_size. This only matches the skb head length
    -    when copy_size is the full packet size.
    +    For non-linear test_run output, bpf_test_finish() derives the linear
    +    data copy length from copy_size - frag_size. This only matches the
    +    linear data length when copy_size is the full packet size.
     
         When userspace provides a short data_out buffer, copy_size is clamped to
         that buffer size. If copy_size is smaller than frag_size, the computed
         length becomes negative and bpf_test_finish() returns -ENOSPC before
         copying the packet prefix or updating data_size_out.
     
    -    Compute the linear head length from the skb layout instead, and clamp the
    -    head copy length to copy_size. This preserves the expected partial-copy
    -    semantics: return -ENOSPC, copy the packet prefix that fits in data_out,
    -    and report the full packet length through data_size_out.
    +    Compute the linear data length from the packet layout instead, and clamp
    +    the linear copy length to copy_size. This preserves the expected
    +    partial-copy semantics: return -ENOSPC, copy the packet prefix that fits
    +    in data_out, and report the full packet length through data_size_out.
     
    -    Fixes: 838baa351cee ("bpf: Craft non-linear skbs in BPF_PROG_TEST_RUN")
    +    Fixes: 7855e0db150ad ("bpf: test_run: add xdp_shared_info pointer in bpf_test_finish signature")
         Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
     
      ## net/bpf/test_run.c ##
2:  663847520f0b < -:  ------------ selftests/bpf: Cover partial copy of non-linear skb test_run output
-:  ------------ > 2:  680506532d97 selftests/bpf: Cover partial copy of non-linear test_run output
-- 
2.43.0

[PATCH bpf v2 1/2] bpf: Fix partial copy of non-linear test_run output

[Sun Jian]

For non-linear test_run output, bpf_test_finish() derives the linear
data copy length from copy_size - frag_size. This only matches the
linear data length when copy_size is the full packet size.

When userspace provides a short data_out buffer, copy_size is clamped to
that buffer size. If copy_size is smaller than frag_size, the computed
length becomes negative and bpf_test_finish() returns -ENOSPC before
copying the packet prefix or updating data_size_out.

Compute the linear data length from the packet layout instead, and clamp
the linear copy length to copy_size. This preserves the expected
partial-copy semantics: return -ENOSPC, copy the packet prefix that fits
in data_out, and report the full packet length through data_size_out.

Fixes: 7855e0db150ad ("bpf: test_run: add xdp_shared_info pointer in bpf_test_finish signature")
Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
---
 net/bpf/test_run.c | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
index 2bc04feadfab..976e8fa31bc9 100644
--- a/net/bpf/test_run.c
+++ b/net/bpf/test_run.c
@@ -453,19 +453,16 @@ static int bpf_test_finish(const union bpf_attr *kattr,
 	}
 
 	if (data_out) {
-		int len = sinfo ? copy_size - frag_size : copy_size;
-
-		if (len < 0) {
-			err = -ENOSPC;
-			goto out;
-		}
+		u32 head_len = size - frag_size;
+		u32 len = min(copy_size, head_len);
 
 		if (copy_to_user(data_out, data, len))
 			goto out;
 
 		if (sinfo) {
-			int i, offset = len;
+			u32 offset = len;
 			u32 data_len;
+			int i;
 
 			for (i = 0; i < sinfo->nr_frags; i++) {
 				skb_frag_t *frag = &sinfo->frags[i];
-- 
2.43.0

[PATCH bpf v2 2/2] selftests/bpf: Cover partial copy of non-linear test_run output

[Sun Jian]

prog_run_opts already verifies that BPF_PROG_TEST_RUN returns -ENOSPC
for a short data_out buffer while still reporting the full output size
through data_size_out.

Add the same coverage for non-linear test_run output. Use pass-through
TC and XDP programs with a 9000-byte packet, a 64-byte linear data area,
and a 100-byte data_out buffer. The expected output spans both the linear
data and the first fragment.

Verify that test_run returns -ENOSPC, reports the full packet length
through data_size_out, and copies the packet prefix into data_out for
both non-linear skb and XDP frags paths.

Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
---
 .../selftests/bpf/prog_tests/prog_run_opts.c  | 72 +++++++++++++++++++
 .../selftests/bpf/progs/test_pkt_access.c     | 12 ++++
 2 files changed, 84 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/prog_run_opts.c b/tools/testing/selftests/bpf/prog_tests/prog_run_opts.c
index 01f1d1b6715a..71af1ff02023 100644
--- a/tools/testing/selftests/bpf/prog_tests/prog_run_opts.c
+++ b/tools/testing/selftests/bpf/prog_tests/prog_run_opts.c
@@ -4,6 +4,10 @@
 
 #include "test_pkt_access.skel.h"
 
+#define NONLINEAR_PKT_LEN 9000
+#define NONLINEAR_LINEAR_DATA_LEN 64
+#define SHORT_OUT_LEN 100
+
 static const __u32 duration;
 
 static void check_run_cnt(int prog_fd, __u64 run_cnt)
@@ -20,6 +24,71 @@ static void check_run_cnt(int prog_fd, __u64 run_cnt)
 	      "incorrect number of repetitions, want %llu have %llu\n", run_cnt, info.run_cnt);
 }
 
+static void init_pkt(__u8 *pkt, size_t len)
+{
+	size_t i;
+
+	for (i = 0; i < len; i++)
+		pkt[i] = i & 0xff;
+}
+
+static void test_skb_nonlinear_data_out_partial(struct test_pkt_access *skel)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, topts);
+	__u8 pkt[NONLINEAR_PKT_LEN];
+	__u8 out[SHORT_OUT_LEN];
+	struct __sk_buff skb = {};
+	int prog_fd, err;
+
+	init_pkt(pkt, sizeof(pkt));
+	memset(out, 0xa5, sizeof(out));
+
+	skb.data_end = NONLINEAR_LINEAR_DATA_LEN;
+
+	topts.data_in = pkt;
+	topts.data_size_in = sizeof(pkt);
+	topts.data_out = out;
+	topts.data_size_out = sizeof(out);
+	topts.ctx_in = &skb;
+	topts.ctx_size_in = sizeof(skb);
+
+	prog_fd = bpf_program__fd(skel->progs.tc_pass_prog);
+	err = bpf_prog_test_run_opts(prog_fd, &topts);
+
+	ASSERT_EQ(err, -ENOSPC, "skb_nonlinear_partial_err");
+	ASSERT_EQ(topts.data_size_out, sizeof(pkt), "skb_nonlinear_partial_data_size_out");
+	ASSERT_OK(memcmp(out, pkt, sizeof(out)), "skb_nonlinear_partial_data_out");
+}
+
+static void test_xdp_nonlinear_data_out_partial(struct test_pkt_access *skel)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, topts);
+	__u8 pkt[NONLINEAR_PKT_LEN];
+	__u8 out[SHORT_OUT_LEN];
+	struct xdp_md ctx = {};
+	int prog_fd, err;
+
+	init_pkt(pkt, sizeof(pkt));
+	memset(out, 0xa5, sizeof(out));
+
+	ctx.data = 0;
+	ctx.data_end = NONLINEAR_LINEAR_DATA_LEN;
+
+	topts.data_in = pkt;
+	topts.data_size_in = sizeof(pkt);
+	topts.data_out = out;
+	topts.data_size_out = sizeof(out);
+	topts.ctx_in = &ctx;
+	topts.ctx_size_in = sizeof(ctx);
+
+	prog_fd = bpf_program__fd(skel->progs.xdp_frags_pass_prog);
+	err = bpf_prog_test_run_opts(prog_fd, &topts);
+
+	ASSERT_EQ(err, -ENOSPC, "xdp_nonlinear_partial_err");
+	ASSERT_EQ(topts.data_size_out, sizeof(pkt), "xdp_nonlinear_partial_data_size_out");
+	ASSERT_OK(memcmp(out, pkt, sizeof(out)), "xdp_nonlinear_partial_data_out");
+}
+
 void test_prog_run_opts(void)
 {
 	struct test_pkt_access *skel;
@@ -69,6 +138,9 @@ void test_prog_run_opts(void)
 	run_cnt += topts.repeat;
 	check_run_cnt(prog_fd, run_cnt);
 
+	test_skb_nonlinear_data_out_partial(skel);
+	test_xdp_nonlinear_data_out_partial(skel);
+
 cleanup:
 	if (skel)
 		test_pkt_access__destroy(skel);
diff --git a/tools/testing/selftests/bpf/progs/test_pkt_access.c b/tools/testing/selftests/bpf/progs/test_pkt_access.c
index bce7173152c6..cd284401eebd 100644
--- a/tools/testing/selftests/bpf/progs/test_pkt_access.c
+++ b/tools/testing/selftests/bpf/progs/test_pkt_access.c
@@ -150,3 +150,15 @@ int test_pkt_access(struct __sk_buff *skb)
 
 	return TC_ACT_UNSPEC;
 }
+
+SEC("tc")
+int tc_pass_prog(struct __sk_buff *skb)
+{
+	return TC_ACT_OK;
+}
+
+SEC("xdp.frags")
+int xdp_frags_pass_prog(struct xdp_md *ctx)
+{
+	return XDP_PASS;
+}
-- 
2.43.0