[bpf-next,v6,1/2] bpf: Add bpf_copy_from_user_str kfunc

Commit Message

Jordan Rome Aug. 18, 2024, 12:23 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.

Signed-off-by: Jordan Rome <[e-mail]>
---
 include/uapi/linux/bpf.h       |  8 +++++++
 kernel/bpf/helpers.c           | 44 ++++++++++++++++++++++++++++++++++
 tools/include/uapi/linux/bpf.h |  8 +++++++
 3 files changed, 60 insertions(+)

--
2.43.5

Comments

Andrii Nakryiko Aug. 19, 2024, 10:41 p.m. UTC | #1

On Sat, Aug 17, 2024 at 5:24 PM 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.
>
> Signed-off-by: Jordan Rome <[e-mail]>
> ---
>  include/uapi/linux/bpf.h       |  8 +++++++
>  kernel/bpf/helpers.c           | 44 ++++++++++++++++++++++++++++++++++
>  tools/include/uapi/linux/bpf.h |  8 +++++++
>  3 files changed, 60 insertions(+)
>

LGTM overall, with the issues pointed out below:

Acked-by: Andrii Nakryiko <[e-mail]>

> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index e05b39e39c3f..5e6be3489e43 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -7513,4 +7513,12 @@ struct bpf_iter_num {
>         __u64 __opaque[1];
>  } __attribute__((aligned(8)));
>
> +/*
> + * Flags to control bpf_copy_from_user_str() behaviour.
> + *     - BPF_F_PAD_ZEROS: Memset 0 the tail of the destination buffer on success

I suspect we might want to reuse this flag for similar kfuncs/helpers
in the future, so I'd generalize description a bit. How about
something like:

BPF_F_PAD_ZEROS: pad destination buffer with zeros. (See respective
helpers documentation for exact 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..a0d2cc8f4f3f 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2939,6 +2939,49 @@ __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(!dst__szk))
> +               return 0;
> +
> +       if (unlikely(flags & ~BPF_F_PAD_ZEROS))
> +               return -EINVAL;
> +

let's move this up before dst__szk check, invalid flags should be
rejected regardless of dst__szk

pw-bot: cr

> +       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';
> +
> +       ret++;
> +
> +       return ret;

nit: return ret + 1;


> +}
> +
>  __bpf_kfunc_end_defs();
>
>  BTF_KFUNCS_START(generic_btf_ids)
> @@ -3024,6 +3067,7 @@ 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..a8dcb99ed904 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -7513,4 +7513,12 @@ struct bpf_iter_num {
>         __u64 __opaque[1];
>  } __attribute__((aligned(8)));
>
> +/*
> + * Flags to control bpf_copy_from_user_str() behaviour.
> + *     - BPF_F_PAD_ZEROS: Memset 0 the entire destination buffer on success
> + */
> +enum {
> +       BPF_F_PAD_ZEROS = (1ULL << 0)
> +};
> +
>  #endif /* _UAPI__LINUX_BPF_H__ */
> --
> 2.43.5
>

Patch

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index e05b39e39c3f..5e6be3489e43 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -7513,4 +7513,12 @@  struct bpf_iter_num {
 	__u64 __opaque[1];
 } __attribute__((aligned(8)));

+/*
+ * Flags to control bpf_copy_from_user_str() behaviour.
+ *     - BPF_F_PAD_ZEROS: Memset 0 the tail of the destination buffer on success
+ */
+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..a0d2cc8f4f3f 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2939,6 +2939,49 @@  __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(!dst__szk))
+		return 0;
+
+	if (unlikely(flags & ~BPF_F_PAD_ZEROS))
+		return -EINVAL;
+
+	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';
+
+	ret++;
+
+	return ret;
+}
+
 __bpf_kfunc_end_defs();

 BTF_KFUNCS_START(generic_btf_ids)
@@ -3024,6 +3067,7 @@  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..a8dcb99ed904 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -7513,4 +7513,12 @@  struct bpf_iter_num {
 	__u64 __opaque[1];
 } __attribute__((aligned(8)));

+/*
+ * Flags to control bpf_copy_from_user_str() behaviour.
+ *     - BPF_F_PAD_ZEROS: Memset 0 the entire destination buffer on success
+ */
+enum {
+	BPF_F_PAD_ZEROS = (1ULL << 0)
+};
+
 #endif /* _UAPI__LINUX_BPF_H__ */