[PATCH bpf-next v4 1/2] bpf: add bpf_strcat,bpf_strncat kfunc

[Rong Tao]

From: Rong Tao <rongtao@xxxxxxxx>

Add string concatenation kfuncs, prototype:

  int bpf_strcat(char *dst, u32 dst__sz, const char *src__ign);
  int bpf_strncat(char *dst, u32 dst__sz, const char *src__ign, u32 len);

This differs from the glibc library functions strcat and strncat, which,
for safety reasons, require the size of the target string's memory space
as a parameter.

The src parameter cannot directly pass a string constant, such as "XYZ",
otherwise it will throw an error "write into map forbidden".

Signed-off-by: Rong Tao <rongtao@xxxxxxxx>
---
 kernel/bpf/helpers.c | 70 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index c18f1e16edee..2c09044de311 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -4195,6 +4195,74 @@ __bpf_kfunc int bpf_strncasestr(const char *s1__ign, const char *s2__ign,
 	return __bpf_strnstr(s1__ign, s2__ign, len, true);
 }
 
+static int __bpf_strncat(char *dst, u32 dsz, const char *src, u32 sz)
+{
+	int dlen = 0, space, copied;
+
+	while (dst[dlen] != '\0')
+		dlen++;
+
+	if (dlen >= dsz || sz == 0 || dsz == 0)
+		return -EINVAL;
+
+	space = dsz - dlen;
+	if (space < 1)
+		return -E2BIG;
+
+	copied = strncpy_from_kernel_nofault(dst + dlen, src, min(sz, space));
+	if (copied < 0)
+		return copied;
+	else if (copied == 0 || copied == 1)
+		return dlen;
+
+	/* The copied character count includes '\0'. */
+	return dlen + copied - 1;
+}
+
+/**
+ * bpf_strcat - Append non-null bytes from a source string, and null-terminate
+ *              the result
+ * @dst: Destination string.
+ * @dst__sz: Maximum bytes of @dst__ign, includes the trailing NUL.
+ * @src: Source string.
+ *
+ * Return:
+ * * >=0      - Length of the concatenated string.
+ *
+ * * %-EINVAL - String @dst__ign is invalid.
+ * * %-EFAULT - Cannot read or write one of the strings.
+ * * %-E2BIG  - String @src__ign is too large or the remaining space in
+ *              @dst__ign is too small.
+ * * %-ERANGE - One of the strings is outside of kernel address space
+ */
+__bpf_kfunc int bpf_strcat(char *dst, u32 dst__sz, const char *src)
+{
+	return __bpf_strncat(dst, dst__sz, src, dst__sz);
+}
+
+/**
+ * bpf_strncat - Append non-null bytes from a source string, and null-terminate
+ *               the result
+ * @dst: Destination string.
+ * @dst__sz: Maximum bytes of @dst__ign, includes the trailing NUL.
+ * @src: Source string.
+ * @len: the maximum number of characters to concatenate, the null terminator
+ *       \0 of the target string @dst will also be included.
+ *
+ * Return:
+ * * >=0      - Length of the concatenated string.
+ *
+ * * %-EINVAL - String @dst__ign is invalid.
+ * * %-EFAULT - Cannot read or write one of the strings.
+ * * %-E2BIG  - String @src__ign is too large or the remaining space in
+ *              @dst__ign is too small.
+ * * %-ERANGE - One of the strings is outside of kernel address space
+ */
+__bpf_kfunc int bpf_strncat(char *dst, u32 dst__sz, const char *src, u32 len)
+{
+	return __bpf_strncat(dst, dst__sz, src, len);
+}
+
 #ifdef CONFIG_KEYS
 /**
  * bpf_lookup_user_key - lookup a key by its serial
@@ -4958,6 +5026,8 @@ BTF_ID_FLAGS(func, bpf_strstr);
 BTF_ID_FLAGS(func, bpf_strcasestr);
 BTF_ID_FLAGS(func, bpf_strnstr);
 BTF_ID_FLAGS(func, bpf_strncasestr);
+BTF_ID_FLAGS(func, bpf_strcat);
+BTF_ID_FLAGS(func, bpf_strncat);
 #if defined(CONFIG_BPF_LSM) && defined(CONFIG_CGROUPS)
 BTF_ID_FLAGS(func, bpf_cgroup_read_xattr, KF_RCU)
 #endif
-- 
2.55.0