[bpf-next,v7,1/2] bpf: Add bpf_copy_from_user_str kfunc

Commit Message

Jordan Rome Aug. 20, 2024, 10:17 a.m. UTC

This adds a kfunc wrapper around strncpy_from_user,
which can be called from sleepable BPF programs.

This matches the non-sleepable 'bpf_probe_read_user_str'
helper except it includes an additional 'flags'
param, which allows consumers to clear the entire
destination buffer on success or failure.

Signed-off-by: Jordan Rome <[e-mail]>
---
 include/uapi/linux/bpf.h       |  9 ++++++++
 kernel/bpf/helpers.c           | 42 ++++++++++++++++++++++++++++++++++
 tools/include/uapi/linux/bpf.h |  9 ++++++++
 3 files changed, 60 insertions(+)

--
2.43.5

Comments

Alexei Starovoitov Aug. 22, 2024, 10:20 p.m. UTC | #1

On Tue, Aug 20, 2024 at 3:18 AM Jordan Rome <[e-mail]> wrote:
>
> This adds a kfunc wrapper around strncpy_from_user,
> which can be called from sleepable BPF programs.
>
> This matches the non-sleepable 'bpf_probe_read_user_str'
> helper except it includes an additional 'flags'
> param, which allows consumers to clear the entire
> destination buffer on success or failure.
>
> Signed-off-by: Jordan Rome <[e-mail]>
> ---
>  include/uapi/linux/bpf.h       |  9 ++++++++
>  kernel/bpf/helpers.c           | 42 ++++++++++++++++++++++++++++++++++
>  tools/include/uapi/linux/bpf.h |  9 ++++++++
>  3 files changed, 60 insertions(+)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index e05b39e39c3f..d3b69cb055c0 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -7513,4 +7513,13 @@ struct bpf_iter_num {
>         __u64 __opaque[1];
>  } __attribute__((aligned(8)));
>
> +/*
> + * Flags to control bpf_copy_from_user_str() behaviour.
> + *     - BPF_F_PAD_ZEROS: Pad destination buffer with zeros. (See the respective
> + *       helper documentation for details.)
> + */
> +enum {
> +       BPF_F_PAD_ZEROS = (1ULL << 0)

Pls add , so this line doesn't have to be touched in the future.

> +++ b/tools/include/uapi/linux/bpf.h
> +enum {
> +       BPF_F_PAD_ZEROS = (1ULL << 0)

and here, of course, as well.

> +__bpf_kfunc int bpf_copy_from_user_str(void *dst, u32 dst__szk, const void __user *unsafe_ptr__ign, u64 flags)

'k' in __szk is probably too restrictive.
It should work with __sz.
The difference is 'k' requires absolute constant
while __sz is ok with bounded value.
Probably worth adding a test as well.

Patch

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index e05b39e39c3f..d3b69cb055c0 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -7513,4 +7513,13 @@  struct bpf_iter_num {
 	__u64 __opaque[1];
 } __attribute__((aligned(8)));

+/*
+ * Flags to control bpf_copy_from_user_str() behaviour.
+ *     - BPF_F_PAD_ZEROS: Pad destination buffer with zeros. (See the respective
+ *       helper documentation for details.)
+ */
+enum {
+	BPF_F_PAD_ZEROS = (1ULL << 0)
+};
+
 #endif /* _UAPI__LINUX_BPF_H__ */
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index d02ae323996b..e5614a980d59 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2939,6 +2939,47 @@  __bpf_kfunc void bpf_iter_bits_destroy(struct bpf_iter_bits *it)
 	bpf_mem_free(&bpf_global_ma, kit->bits);
 }

+/**
+ * bpf_copy_from_user_str() - Copy a string from an unsafe user address
+ * @dst:             Destination address, in kernel space.  This buffer must be at
+ *                   least @dst__szk bytes long.
+ * @dst__szk:        Maximum number of bytes to copy, including the trailing NUL.
+ * @unsafe_ptr__ign: Source address, in user space.
+ * @flags:           The only supported flag is BPF_F_PAD_ZEROS
+ *
+ * Copies a NUL-terminated string from userspace to BPF space. If user string is
+ * too long this will still ensure zero termination in the dst buffer unless
+ * buffer size is 0.
+ *
+ * If BPF_F_PAD_ZEROS flag is set, memset the tail of @dst to 0 on success and
+ * memset all of @dst on failure.
+ */
+__bpf_kfunc int bpf_copy_from_user_str(void *dst, u32 dst__szk, const void __user *unsafe_ptr__ign, u64 flags)
+{
+	int ret;
+
+	if (unlikely(flags & ~BPF_F_PAD_ZEROS))
+		return -EINVAL;
+
+	if (unlikely(!dst__szk))
+		return 0;
+
+	ret = strncpy_from_user(dst, unsafe_ptr__ign, dst__szk - 1);
+	if (ret < 0) {
+		if (flags & BPF_F_PAD_ZEROS)
+			memset((char *)dst, 0, dst__szk);
+
+		return ret;
+	}
+
+	if (flags & BPF_F_PAD_ZEROS)
+		memset((char *)dst + ret, 0, dst__szk - ret);
+	else
+		((char *)dst)[ret] = '\0';
+
+	return ret + 1;
+}
+
 __bpf_kfunc_end_defs();

 BTF_KFUNCS_START(generic_btf_ids)
@@ -3024,6 +3065,7 @@  BTF_ID_FLAGS(func, bpf_preempt_enable)
 BTF_ID_FLAGS(func, bpf_iter_bits_new, KF_ITER_NEW)
 BTF_ID_FLAGS(func, bpf_iter_bits_next, KF_ITER_NEXT | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_iter_bits_destroy, KF_ITER_DESTROY)
+BTF_ID_FLAGS(func, bpf_copy_from_user_str, KF_SLEEPABLE)
 BTF_KFUNCS_END(common_btf_ids)

 static const struct btf_kfunc_id_set common_kfunc_set = {
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index e05b39e39c3f..d3b69cb055c0 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -7513,4 +7513,13 @@  struct bpf_iter_num {
 	__u64 __opaque[1];
 } __attribute__((aligned(8)));

+/*
+ * Flags to control bpf_copy_from_user_str() behaviour.
+ *     - BPF_F_PAD_ZEROS: Pad destination buffer with zeros. (See the respective
+ *       helper documentation for details.)
+ */
+enum {
+	BPF_F_PAD_ZEROS = (1ULL << 0)
+};
+
 #endif /* _UAPI__LINUX_BPF_H__ */