[PATCH bpf] bpf: roll back stream capacity when allocation fails

[PATCH bpf] bpf: roll back stream capacity when allocation fails

[Jianlin Shi]

bpf_stream_push_str() accounts the string length before allocating a
stream element. If the allocation fails, the length remains charged even
though no element is queued and therefore cannot be released by a reader.
Repeated failures can exhaust the stream capacity permanently until the
BPF program is freed.

Roll back the capacity charge when creating the stream element fails.

Fixes: 5ab154f1463a ("bpf: Introduce BPF standard streams")
Signed-off-by: Jianlin Shi <shijianlin11@foxmail.com>
---
 kernel/bpf/stream.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/stream.c b/kernel/bpf/stream.c
index be9ce98e9..4b8a74b91 100644
--- a/kernel/bpf/stream.c
+++ b/kernel/bpf/stream.c
@@ -79,7 +79,14 @@ static int bpf_stream_push_str(struct bpf_stream *stream, const char *str, int l
 {
 	int ret = bpf_stream_consume_capacity(stream, len);
 
-	return ret ?: __bpf_stream_push_str(&stream->log, str, len);
+	if (ret)
+		return ret;
+
+	ret = __bpf_stream_push_str(&stream->log, str, len);
+	if (ret)
+		atomic_sub(len, &stream->capacity);
+
+	return ret;
 }
 
 static struct bpf_stream *bpf_stream_get(enum bpf_stream_id stream_id, struct bpf_prog_aux *aux)
-- 
2.43.0

Re: [PATCH bpf] bpf: roll back stream capacity when allocation fails

[Pu Lehui]

On 2026/7/20 9:23, Jianlin Shi wrote:
> bpf_stream_push_str() accounts the string length before allocating a
> stream element. If the allocation fails, the length remains charged even
> though no element is queued and therefore cannot be released by a reader.
> Repeated failures can exhaust the stream capacity permanently until the
> BPF program is freed.
> 
> Roll back the capacity charge when creating the stream element fails.
> 
> Fixes: 5ab154f1463a ("bpf: Introduce BPF standard streams")
> Signed-off-by: Jianlin Shi <shijianlin11@foxmail.com>
> ---
>   kernel/bpf/stream.c | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/stream.c b/kernel/bpf/stream.c
> index be9ce98e9..4b8a74b91 100644
> --- a/kernel/bpf/stream.c
> +++ b/kernel/bpf/stream.c
> @@ -79,7 +79,14 @@ static int bpf_stream_push_str(struct bpf_stream *stream, const char *str, int l
>   {
>   	int ret = bpf_stream_consume_capacity(stream, len);
>   
> -	return ret ?: __bpf_stream_push_str(&stream->log, str, len);
> +	if (ret)
> +		return ret;
> +
> +	ret = __bpf_stream_push_str(&stream->log, str, len);
> +	if (ret)
> +		atomic_sub(len, &stream->capacity);
> +
> +	return ret;
>   }
>   
>   static struct bpf_stream *bpf_stream_get(enum bpf_stream_id stream_id, struct bpf_prog_aux *aux)

looks good to me, but it might be better to target bpf-next, as this 
isn't a critical issue.

Re: [PATCH bpf] bpf: roll back stream capacity when allocation fails

[Jianlin Shi]

On 2026/7/20 11:40, Pu Lehui wrote:
> looks good to me, but it might be better to target bpf-next, as this
> isn't a critical issue.

Thanks for the review. Will resend as v2 targeting bpf-next.

Thanks,
Jianlin

[PATCH bpf-next v2] bpf: roll back stream capacity when allocation fails

[Jianlin Shi]

bpf_stream_push_str() accounts the string length before allocating a
stream element. If the allocation fails, the length remains charged even
though no element is queued and therefore cannot be released by a reader.
Repeated failures can exhaust the stream capacity permanently until the
BPF program is freed.

Roll back the capacity charge when creating the stream element fails.

Fixes: 5ab154f1463a ("bpf: Introduce BPF standard streams")
Signed-off-by: Jianlin Shi <shijianlin11@foxmail.com>
---
v2:
- Retarget to bpf-next as suggested by Pu Lehui.

 kernel/bpf/stream.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/stream.c b/kernel/bpf/stream.c
index be9ce98e9..4b8a74b91 100644
--- a/kernel/bpf/stream.c
+++ b/kernel/bpf/stream.c
@@ -79,7 +79,14 @@ static int bpf_stream_push_str(struct bpf_stream *stream, const char *str, int l
 {
 	int ret = bpf_stream_consume_capacity(stream, len);
 
-	return ret ?: __bpf_stream_push_str(&stream->log, str, len);
+	if (ret)
+		return ret;
+
+	ret = __bpf_stream_push_str(&stream->log, str, len);
+	if (ret)
+		atomic_sub(len, &stream->capacity);
+
+	return ret;
 }
 
 static struct bpf_stream *bpf_stream_get(enum bpf_stream_id stream_id, struct bpf_prog_aux *aux)
-- 
2.43.0

Re: [PATCH bpf-next v2] bpf: roll back stream capacity when allocation fails

[Jianlin Shi]

Friendly ping on v2.

Regarding the related capacity leak in bpf_stream_stage_printk(),
I can send that as a separate follow-up once this patch is settled.

Thanks,
Jianlin

Re: [PATCH bpf-next v2] bpf: roll back stream capacity when allocation fails

[Pu Lehui]

On 2026/7/20 13:13, Jianlin Shi wrote:
> bpf_stream_push_str() accounts the string length before allocating a
> stream element. If the allocation fails, the length remains charged even
> though no element is queued and therefore cannot be released by a reader.
> Repeated failures can exhaust the stream capacity permanently until the
> BPF program is freed.
> 
> Roll back the capacity charge when creating the stream element fails.
> 
> Fixes: 5ab154f1463a ("bpf: Introduce BPF standard streams")
> Signed-off-by: Jianlin Shi <shijianlin11@foxmail.com>
> ---
> v2:
> - Retarget to bpf-next as suggested by Pu Lehui.
> 
>   kernel/bpf/stream.c | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/stream.c b/kernel/bpf/stream.c
> index be9ce98e9..4b8a74b91 100644
> --- a/kernel/bpf/stream.c
> +++ b/kernel/bpf/stream.c
> @@ -79,7 +79,14 @@ static int bpf_stream_push_str(struct bpf_stream *stream, const char *str, int l
>   {
>   	int ret = bpf_stream_consume_capacity(stream, len);
>   
> -	return ret ?: __bpf_stream_push_str(&stream->log, str, len);
> +	if (ret)
> +		return ret;
> +
> +	ret = __bpf_stream_push_str(&stream->log, str, len);
> +	if (ret)
> +		atomic_sub(len, &stream->capacity);
> +
> +	return ret;
>   }
>   
>   static struct bpf_stream *bpf_stream_get(enum bpf_stream_id stream_id, struct bpf_prog_aux *aux)


Reviewed-by: Pu Lehui <pulehui@huawei.com>

Re: [PATCH bpf-next v2] bpf: roll back stream capacity when allocation fails

[Kumar Kartikeya Dwivedi]

On Mon Jul 20, 2026 at 7:13 AM CEST, Jianlin Shi wrote:
> bpf_stream_push_str() accounts the string length before allocating a
> stream element. If the allocation fails, the length remains charged even
> though no element is queued and therefore cannot be released by a reader.
> Repeated failures can exhaust the stream capacity permanently until the
> BPF program is freed.
>
> Roll back the capacity charge when creating the stream element fails.
>
> Fixes: 5ab154f1463a ("bpf: Introduce BPF standard streams")
> Signed-off-by: Jianlin Shi <shijianlin11@foxmail.com>
> ---

Sorry about the delay. This particular fix is correct, but I'd prefer if we
could refactor bpf_stream_release_capacity() to accept length as its second
parameter and then use it here, to drop dependency on 'elem'. That also better
mirrors the consume side.

As for other Sashiko concerns, since you will respin, you can append more
patches to the series.

Regarding the three issues pointed out by Sashiko in reply to v2, they all look
valid to me. For the staging leak, ss->len should only be increased after
__bpf_stream_push_str() succeeds. We shoulf fix that.

On the read side, account for any bytes successfully copied before a fault and
return that byte count, returning -EFAULT only when no bytes were copied. So a
partial buffer which allows copying some bytes successfully and fails the rest,
we should return the partial count of bytes copied.

For oversized formatted output, keeping the current behavior of dropping the
message seems reasonable, but -ENOMEM is misleading. Since the formatter's
return value excludes the trailing NUL and a value greater than or equal to
MAX_BPRINTF_BUF indicates truncation, please reject such lengths with -E2BIG,
preferably before charging stream capacity. This also fixes the boundary case
where a length of MAX_BPRINTF_BUF currently copies the trailing NUL into the
stream.

So use the actual formatted length, capped at MAX_BPRINTF_BUF - 1 (vscnprintf()
for the staged path), so oversized messages are truncated without copying the
trailing NUL.

Please also add selftests for such cases. For the partial unmapped buffer, you
could map a page and pass a buffer straddling the page boundary, such that we
take a fault when copying into the remainder.

pw-bot: cr

> v2:
> - Retarget to bpf-next as suggested by Pu Lehui.
>
>  kernel/bpf/stream.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/stream.c b/kernel/bpf/stream.c
> index be9ce98e9..4b8a74b91 100644
> --- a/kernel/bpf/stream.c
> +++ b/kernel/bpf/stream.c
> @@ -79,7 +79,14 @@ static int bpf_stream_push_str(struct bpf_stream *stream, const char *str, int l
>  {
>  	int ret = bpf_stream_consume_capacity(stream, len);
>
> -	return ret ?: __bpf_stream_push_str(&stream->log, str, len);
> +	if (ret)
> +		return ret;
> +
> +	ret = __bpf_stream_push_str(&stream->log, str, len);
> +	if (ret)
> +		atomic_sub(len, &stream->capacity);
> +
> +	return ret;
>  }
>
>  static struct bpf_stream *bpf_stream_get(enum bpf_stream_id stream_id, struct bpf_prog_aux *aux)

Re: [PATCH bpf-next v2] bpf: roll back stream capacity when allocation fails

[Jianlin Shi]

On Mon, 03 Aug 2026 04:40:50 +0200 Kumar Kartikeya Dwivedi wrote:
> Sorry about the delay. This particular fix is correct, but I'd prefer if we
> could refactor bpf_stream_release_capacity() to accept length as its second
> parameter and then use it here, to drop dependency on 'elem'. That also better
> mirrors the consume side.
>
> As for other Sashiko concerns, since you will respin, you can append more
> patches to the series.
>
> [...]
>
> pw-bot: cr

Thanks for the review.  I will address the feedback, including the
release_capacity() refactor and the additional issues as follow-up
patches, and send a v3 series shortly.

Thanks,
Jianlin

[PATCH bpf-next v3 0/5] bpf: fix stream capacity, read, and oversize handling

[Jianlin Shi]

v2 of the capacity-rollback fix was correct for bpf_stream_push_str(),
but review asked to:

1. refactor bpf_stream_release_capacity() to take a length;
2. fix the staging-path capacity leak and use vscnprintf();
3. return partial bpf_stream_read() progress on copy_to_user() fault;
4. reject truncated bpf_stream_vprintk() output with -E2BIG before
   charging capacity;
5. add selftests for the oversize and straddling-buffer cases.

This series addresses those points.

Tested locally:
  stream_oversize and stream_partial_read (equivalent to
  ./test_progs -t stream_oversize,stream_partial_read).

v2: https://lore.kernel.org/bpf/tencent_C919BB32458A4DAD645A68F441345B971E05@qq.com/
v1: https://lore.kernel.org/bpf/tencent_E69EAE29327E25B3548A9AF3F4FA289A6806@qq.com/

Jianlin Shi (5):
  bpf: roll back stream capacity when allocation fails
  bpf: fix stream capacity leak in staging path
  bpf: return partial progress from bpf_stream_read on fault
  bpf: reject oversized bpf_stream_vprintk output with -E2BIG
  selftests/bpf: cover stream capacity and partial read edge cases

 kernel/bpf/stream.c                           | 54 ++++++++------
 .../testing/selftests/bpf/prog_tests/stream.c | 70 +++++++++++++++++++
 tools/testing/selftests/bpf/progs/stream.c    | 12 ++++
 3 files changed, 116 insertions(+), 20 deletions(-)

-- 
2.43.0

[PATCH bpf-next v3 1/5] bpf: roll back stream capacity when allocation fails

[Jianlin Shi]

bpf_stream_push_str() accounts the string length before allocating a
stream element. If the allocation fails, the length remains charged even
though no element is queued and therefore cannot be released by a reader.
Repeated failures can exhaust the stream capacity permanently until the
BPF program is freed.

Refactor bpf_stream_release_capacity() to take a length so the consume
and release sides are symmetric, and use it to roll back the charge when
creating the stream element fails.

Fixes: 5ab154f1463a ("bpf: Introduce BPF standard streams")
Signed-off-by: Jianlin Shi <shijianlin11@foxmail.com>
---
 kernel/bpf/stream.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/stream.c b/kernel/bpf/stream.c
index be9ce98e9469..0b157ec4e38e 100644
--- a/kernel/bpf/stream.c
+++ b/kernel/bpf/stream.c
@@ -68,10 +68,8 @@ static int bpf_stream_consume_capacity(struct bpf_stream *stream, int len)
 	return 0;
 }
 
-static void bpf_stream_release_capacity(struct bpf_stream *stream, struct bpf_stream_elem *elem)
+static void bpf_stream_release_capacity(struct bpf_stream *stream, int len)
 {
-	int len = elem->total_len;
-
 	atomic_sub(len, &stream->capacity);
 }
 
@@ -79,7 +77,14 @@ static int bpf_stream_push_str(struct bpf_stream *stream, const char *str, int l
 {
 	int ret = bpf_stream_consume_capacity(stream, len);
 
-	return ret ?: __bpf_stream_push_str(&stream->log, str, len);
+	if (ret)
+		return ret;
+
+	ret = __bpf_stream_push_str(&stream->log, str, len);
+	if (ret)
+		bpf_stream_release_capacity(stream, len);
+
+	return ret;
 }
 
 static struct bpf_stream *bpf_stream_get(enum bpf_stream_id stream_id, struct bpf_prog_aux *aux)
@@ -188,7 +193,7 @@ static int bpf_stream_read(struct bpf_stream *stream, void __user *buf, int len)
 		if (cont)
 			continue;
 		bpf_stream_backlog_pop(stream);
-		bpf_stream_release_capacity(stream, elem);
+		bpf_stream_release_capacity(stream, elem->total_len);
 		bpf_stream_free_elem(elem);
 	}
 
-- 
2.43.0

[PATCH bpf-next v3 2/5] bpf: fix stream capacity leak in staging path

[Jianlin Shi]

bpf_stream_stage_printk() increments ss->len before pushing the
formatted string to the staging log. If element allocation fails,
ss->len remains inflated and bpf_stream_stage_commit() permanently
charges the stream capacity for data that was never queued.

Only account the string length after a successful push, and use
vscnprintf() so the staged length is the truncated payload without
the trailing NUL.

Fixes: 5ab154f1463a ("bpf: Introduce BPF standard streams")
Signed-off-by: Jianlin Shi <shijianlin11@foxmail.com>
---
 kernel/bpf/stream.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/stream.c b/kernel/bpf/stream.c
index 0b157ec4e38e..ef5a7b06ffcb 100644
--- a/kernel/bpf/stream.c
+++ b/kernel/bpf/stream.c
@@ -316,17 +316,19 @@ int bpf_stream_stage_printk(struct bpf_stream_stage *ss, const char *fmt, ...)
 {
 	struct bpf_bprintf_buffers *buf;
 	va_list args;
-	int ret;
+	int len, ret;
 
 	if (bpf_try_get_buffers(&buf))
 		return -EBUSY;
 
 	va_start(args, fmt);
-	ret = vsnprintf(buf->buf, ARRAY_SIZE(buf->buf), fmt, args);
+	/* Cap to the written length, excluding the trailing NUL. */
+	len = vscnprintf(buf->buf, ARRAY_SIZE(buf->buf), fmt, args);
 	va_end(args);
-	ss->len += ret;
 	/* Exclude NULL byte during push. */
-	ret = __bpf_stream_push_str(&ss->log, buf->buf, ret);
+	ret = __bpf_stream_push_str(&ss->log, buf->buf, len);
+	if (!ret)
+		ss->len += len;
 	bpf_put_buffers();
 	return ret;
 }
-- 
2.43.0

Re: [PATCH bpf-next v3 2/5] bpf: fix stream capacity leak in staging path

[bot+bpf-ci]

[-- Attachment #1: Type: text/plain, Size: 1337 bytes --]

> diff --git a/kernel/bpf/stream.c b/kernel/bpf/stream.c
> index 0b157ec4e38e..ef5a7b06ffcb 100644
> --- a/kernel/bpf/stream.c
> +++ b/kernel/bpf/stream.c
> @@ -316,17 +316,19 @@ int bpf_stream_stage_printk(struct bpf_stream_stage *ss, const char *fmt, ...)
>  {
>  	struct bpf_bprintf_buffers *buf;
>  	va_list args;
> -	int ret;
> +	int len, ret;
>
>  	if (bpf_try_get_buffers(&buf))
>  		return -EBUSY;
>
>  	va_start(args, fmt);
> -	ret = vsnprintf(buf->buf, ARRAY_SIZE(buf->buf), fmt, args);
> +	/* Cap to the written length, excluding the trailing NUL. */
> +	len = vscnprintf(buf->buf, ARRAY_SIZE(buf->buf), fmt, args);
>  	va_end(args);
> -	ss->len += ret;
>  	/* Exclude NULL byte during push. */
> -	ret = __bpf_stream_push_str(&ss->log, buf->buf, ret);
> +	ret = __bpf_stream_push_str(&ss->log, buf->buf, len);
> +	if (!ret)
> +		ss->len += len;

This isn't a bug, but with the existing '/* Exclude NULL byte during
push. */' a couple of lines down, does the new comment above vscnprintf()
add anything, or could one of the two be dropped?

>  	bpf_put_buffers();
>  	return ret;
>  }


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/31874860107

[PATCH bpf-next v3 3/5] bpf: return partial progress from bpf_stream_read on fault

[Jianlin Shi]

bpf_stream_read() pops and frees stream elements after a successful
copy_to_user(). If a later copy_to_user() fails, it currently restores
only the current element's consumed_len and returns -EFAULT, hiding
bytes already delivered to userspace and making the consumed data
unrecoverable on retry.

On a short copy, keep the successfully copied prefix of the current
element and return the number of bytes copied. Return -EFAULT only when
no bytes were copied for the call.

Fixes: 5ab154f1463a ("bpf: Introduce BPF standard streams")
Signed-off-by: Jianlin Shi <shijianlin11@foxmail.com>
---
 kernel/bpf/stream.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/kernel/bpf/stream.c b/kernel/bpf/stream.c
index ef5a7b06ffcb..c1077160074c 100644
--- a/kernel/bpf/stream.c
+++ b/kernel/bpf/stream.c
@@ -167,6 +167,7 @@ static int bpf_stream_read(struct bpf_stream *stream, void __user *buf, int len)
 
 	while (rem_len) {
 		int pos = len - rem_len;
+		int chunk, n;
 		bool cont;
 
 		node = bpf_stream_backlog_peek(stream);
@@ -180,13 +181,14 @@ static int bpf_stream_read(struct bpf_stream *stream, void __user *buf, int len)
 
 		cons_len = elem->consumed_len;
 		cont = bpf_stream_consume_elem(elem, &rem_len) == false;
-
-		ret = copy_to_user(buf + pos, elem->str + cons_len,
-				   elem->consumed_len - cons_len);
-		/* Restore in case of error. */
-		if (ret) {
-			ret = -EFAULT;
-			elem->consumed_len = cons_len;
+		chunk = elem->consumed_len - cons_len;
+
+		n = copy_to_user(buf + pos, elem->str + cons_len, chunk);
+		if (n) {
+			/* Keep any successfully copied bytes; -EFAULT only if none. */
+			elem->consumed_len -= n;
+			rem_len += n;
+			ret = (len == rem_len) ? -EFAULT : 0;
 			break;
 		}
 
-- 
2.43.0

[PATCH bpf-next v3 4/5] bpf: reject oversized bpf_stream_vprintk output with -E2BIG

[Jianlin Shi]

bstr_printf() returns the would-be length excluding the trailing NUL.
When that value is >= MAX_BPRINTF_BUF the message was truncated, but
bpf_stream_push_str() still tried to allocate with the inflated length
and failed with -ENOMEM. The boundary case of exactly MAX_BPRINTF_BUF
could also copy the trailing NUL into the stream element.

Reject such lengths with -E2BIG before charging stream capacity, and
tighten bpf_stream_elem_alloc() to accept only payloads strictly shorter
than the bprintf buffer.

Fixes: 5ab154f1463a ("bpf: Introduce BPF standard streams")
Signed-off-by: Jianlin Shi <shijianlin11@foxmail.com>
---
 kernel/bpf/stream.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/stream.c b/kernel/bpf/stream.c
index c1077160074c..bd1e98fde4b0 100644
--- a/kernel/bpf/stream.c
+++ b/kernel/bpf/stream.c
@@ -22,11 +22,11 @@ static struct bpf_stream_elem *bpf_stream_elem_alloc(int len)
 	size_t alloc_size;
 
 	/*
-	 * Length denotes the amount of data to be written as part of stream element,
-	 * thus includes '\0' byte. We're capped by how much bpf_bprintf_buffers can
-	 * accomodate, therefore deny allocations that won't fit into them.
+	 * Length is the payload pushed into the stream, excluding the
+	 * trailing NUL of the bprintf buffer. Reject anything that cannot
+	 * fit without copying that NUL into the stream element.
 	 */
-	if (len < 0 || len > max_len)
+	if (len < 0 || len >= max_len)
 		return NULL;
 
 	alloc_size = offsetof(struct bpf_stream_elem, str[len]);
@@ -245,6 +245,11 @@ __bpf_kfunc int bpf_stream_vprintk(int stream_id, const char *fmt__str, const vo
 		return ret;
 
 	ret = bstr_printf(data.buf, MAX_BPRINTF_BUF, fmt__str, data.bin_args);
+	/* Truncation: reject before capacity charge (not -ENOMEM). */
+	if (ret >= MAX_BPRINTF_BUF) {
+		bpf_bprintf_cleanup(&data);
+		return -E2BIG;
+	}
 	/* Exclude NULL byte during push. */
 	ret = bpf_stream_push_str(stream, data.buf, ret);
 	bpf_bprintf_cleanup(&data);
-- 
2.43.0

[PATCH bpf-next v3 5/5] selftests/bpf: cover stream capacity and partial read edge cases

[Jianlin Shi]

Add coverage for the stream fixes requested on the capacity rollback
series:

- oversized bpf_stream_printk() returns -E2BIG and does not leak
  capacity for a subsequent successful write;
- bpf_prog_stream_read() returns the successfully copied prefix when
  the userspace buffer straddles an unmapped page.

Signed-off-by: Jianlin Shi <shijianlin11@foxmail.com>
---
 .../testing/selftests/bpf/prog_tests/stream.c | 70 +++++++++++++++++++
 tools/testing/selftests/bpf/progs/stream.c    | 12 ++++
 2 files changed, 82 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/stream.c b/tools/testing/selftests/bpf/prog_tests/stream.c
index c3cce5c292bd..3ae98e3c4909 100644
--- a/tools/testing/selftests/bpf/prog_tests/stream.c
+++ b/tools/testing/selftests/bpf/prog_tests/stream.c
@@ -58,6 +58,76 @@ void test_stream_syscall(void)
 	stream__destroy(skel);
 }
 
+void test_stream_oversize(void)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, opts);
+	struct stream *skel;
+	int ret, prog_fd;
+	char buf[8] = {};
+
+	skel = stream__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "stream__open_and_load"))
+		return;
+
+	prog_fd = bpf_program__fd(skel->progs.stream_oversize);
+	ret = bpf_prog_test_run_opts(prog_fd, &opts);
+	ASSERT_OK(ret, "oversize run");
+	ASSERT_EQ(opts.retval, -E2BIG, "oversize retval");
+
+	/* Oversized push must not permanently consume capacity. */
+	prog_fd = bpf_program__fd(skel->progs.stream_syscall);
+	ret = bpf_prog_test_run_opts(prog_fd, &opts);
+	ASSERT_OK(ret, "syscall run");
+	ASSERT_OK(opts.retval, "syscall retval");
+
+	ret = bpf_prog_stream_read(prog_fd, BPF_STREAM_STDOUT, buf, sizeof(buf), NULL);
+	ASSERT_EQ(ret, 3, "bytes after oversize");
+	ASSERT_OK(memcmp(buf, "foo", 3), "payload after oversize");
+
+	stream__destroy(skel);
+}
+
+void test_stream_partial_read(void)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, opts);
+	struct stream *skel;
+	int ret, prog_fd;
+	long page_size;
+	char *page, *buf;
+	char rest[8] = {};
+
+	skel = stream__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "stream__open_and_load"))
+		return;
+
+	prog_fd = bpf_program__fd(skel->progs.stream_syscall);
+	ret = bpf_prog_test_run_opts(prog_fd, &opts);
+	ASSERT_OK(ret, "ret");
+	ASSERT_OK(opts.retval, "retval");
+
+	page_size = sysconf(_SC_PAGESIZE);
+	page = mmap(NULL, page_size * 2, PROT_READ | PROT_WRITE,
+		    MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	if (!ASSERT_NEQ(page, MAP_FAILED, "mmap")) {
+		stream__destroy(skel);
+		return;
+	}
+	/* Leave only the first page mapped so a straddling copy faults. */
+	ASSERT_OK(munmap(page + page_size, page_size), "munmap second page");
+
+	buf = page + page_size - 1;
+	ret = bpf_prog_stream_read(prog_fd, BPF_STREAM_STDOUT, buf, 3, NULL);
+	ASSERT_EQ(ret, 1, "partial bytes");
+	ASSERT_EQ(buf[0], 'f', "first byte");
+
+	ret = bpf_prog_stream_read(prog_fd, BPF_STREAM_STDOUT, rest, sizeof(rest), NULL);
+	ASSERT_EQ(ret, 2, "remaining bytes");
+	ASSERT_OK(memcmp(rest, "oo", 2), "remaining data");
+
+	munmap(page, page_size);
+	stream__destroy(skel);
+}
+
 static void test_address(struct bpf_program *prog, unsigned long *fault_addr_p)
 {
 	LIBBPF_OPTS(bpf_test_run_opts, opts);
diff --git a/tools/testing/selftests/bpf/progs/stream.c b/tools/testing/selftests/bpf/progs/stream.c
index 6f999ba951a3..6fab9b8b21a0 100644
--- a/tools/testing/selftests/bpf/progs/stream.c
+++ b/tools/testing/selftests/bpf/progs/stream.c
@@ -36,7 +36,12 @@ struct {
 } array SEC(".maps");
 
 #define ENOSPC 28
+#define E2BIG 7
 #define _STR "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
+#define _X64 "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
+/* 1024 bytes: truncated by bstr_printf, must return -E2BIG. */
+#define _BIG_STR (_X64 _X64 _X64 _X64 _X64 _X64 _X64 _X64 \
+		  _X64 _X64 _X64 _X64 _X64 _X64 _X64 _X64)
 
 int size;
 u64 fault_addr;
@@ -117,6 +122,13 @@ int stream_syscall(void *ctx)
 	return 0;
 }
 
+SEC("syscall")
+__success __retval(-E2BIG)
+int stream_oversize(void *ctx)
+{
+	return bpf_stream_printk(BPF_STDOUT, _BIG_STR);
+}
+
 SEC("syscall")
 __arch_x86_64
 __arch_arm64
-- 
2.43.0