[bpf-next v3 2/3] bpf: Add bpf_copy_from_user_task_str kfunc

[Jordan Rome <linux@jordanrome.com>]

This new kfunc will be able to copy a string
from another process's/task's address space.
This is similar to `bpf_copy_from_user_str`
but accepts a `struct task_struct*` argument.

Signed-off-by: Jordan Rome <linux@jordanrome.com>
---
 kernel/bpf/helpers.c | 51 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index f27ce162427a..c26fabf97afd 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -3082,6 +3082,56 @@ __bpf_kfunc void bpf_local_irq_restore(unsigned long *flags__irq_flag)
 	local_irq_restore(*flags__irq_flag);
 }

+/**
+ * bpf_copy_from_user_task_str() - Copy a string from an task's address space
+ * @dst:             Destination address, in kernel space.  This buffer must be
+ *                   at least @dst__sz bytes long.
+ * @dst__sz:         Maximum number of bytes to copy, includes the trailing NUL.
+ * @unsafe_ptr__ign: Source address in the task's address space.
+ * @tsk:             The task whose address space will be used
+ * @flags:           The only supported flag is BPF_F_PAD_ZEROS
+ *
+ * Copies a NULL-terminated string from a task's address space to *dst* buffer.
+ * 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.
+ *
+ * Return: The number of copied bytes on success, including the NULL-terminator.
+ * A negative error code on failure.
+ */
+__bpf_kfunc int bpf_copy_from_user_task_str(void *dst,
+					    u32 dst__sz,
+					    const void __user *unsafe_ptr__ign,
+					    struct task_struct *tsk,
+					    u64 flags)
+{
+	int ret = 0;
+
+	if (unlikely(flags & ~BPF_F_PAD_ZEROS))
+		return -EINVAL;
+
+	if (unlikely(!dst__sz))
+		return 0;
+
+	ret = copy_remote_vm_str(tsk, (unsigned long)unsafe_ptr__ign, dst, dst__sz, 0);
+
+	if (ret <= 0) {
+		if (flags & BPF_F_PAD_ZEROS)
+			memset(dst, 0, dst__sz);
+		return ret ?: -EINVAL;
+	}
+
+	if (ret < dst__sz) {
+		if (flags & BPF_F_PAD_ZEROS)
+			memset(dst + ret, 0, dst__sz - ret);
+		return ret + 1;
+	}
+
+	return ret;
+}
+
 __bpf_kfunc_end_defs();

 BTF_KFUNCS_START(generic_btf_ids)
@@ -3174,6 +3224,7 @@ 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_ID_FLAGS(func, bpf_copy_from_user_task_str, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_get_kmem_cache)
 BTF_ID_FLAGS(func, bpf_iter_kmem_cache_new, KF_ITER_NEW | KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_iter_kmem_cache_next, KF_ITER_NEXT | KF_RET_NULL | KF_SLEEPABLE)
--
2.43.5