[PATCH bpf-next v2 1/3] mm: Add copy_remote_mm_str()

[Anastasios Papagiannis <tasos.papagiannnis@gmail.com>]

copy_remote_vm_str() gets the target address space from a struct
task_struct. This does not work for an address space that exists but is
not yet associated with a task_struct, such as the mm held by struct
linux_binprm during exec.

Add copy_remote_mm_str(), which operates directly on a struct mm_struct.

Signed-off-by: Anastasios Papagiannis <tasos.papagiannnis@gmail.com>
---
 include/linux/mm.h |  2 ++
 mm/memory.c        | 26 +++++++++++++++++++++++++-
 mm/nommu.c         | 26 +++++++++++++++++++++++++-
 3 files changed, 52 insertions(+), 2 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 485df9c2dbdd..eede435bf4a3 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3222,6 +3222,8 @@ extern int access_remote_vm(struct mm_struct *mm, unsigned long addr,
 		void *buf, int len, unsigned int gup_flags);
 
 #ifdef CONFIG_BPF_SYSCALL
+extern int copy_remote_mm_str(struct mm_struct *mm, unsigned long addr,
+			      void *buf, int len, unsigned int gup_flags);
 extern int copy_remote_vm_str(struct task_struct *tsk, unsigned long addr,
 			      void *buf, int len, unsigned int gup_flags);
 #endif
diff --git a/mm/memory.c b/mm/memory.c
index 6b8280cfc1db..da0010c4475d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -7218,6 +7218,30 @@ static int __copy_remote_vm_str(struct mm_struct *mm, unsigned long addr,
 	return buf - old_buf;
 }
 
+/**
+ * copy_remote_mm_str - copy a string from a remote address space.
+ * @mm:	the remote address space
+ * @addr:	start address to read from
+ * @buf:	destination buffer
+ * @len:	number of bytes to copy
+ * @gup_flags:	flags modifying lookup behaviour
+ *
+ * The caller must hold a reference on @mm.
+ *
+ * Return: number of bytes copied from @addr (source) to @buf (destination),
+ * not including the trailing NUL. If @len is zero, return 0 without accessing
+ * @buf. Otherwise, @buf is always NUL-terminated. On any error, return
+ * -EFAULT.
+ */
+int copy_remote_mm_str(struct mm_struct *mm, unsigned long addr,
+		       void *buf, int len, unsigned int gup_flags)
+{
+	if (unlikely(len == 0))
+		return 0;
+
+	return __copy_remote_vm_str(mm, addr, buf, len, gup_flags);
+}
+
 /**
  * copy_remote_vm_str - copy a string from another process's address space.
  * @tsk:	the task of the target address space
@@ -7247,7 +7271,7 @@ int copy_remote_vm_str(struct task_struct *tsk, unsigned long addr,
 		return -EFAULT;
 	}
 
-	ret = __copy_remote_vm_str(mm, addr, buf, len, gup_flags);
+	ret = copy_remote_mm_str(mm, addr, buf, len, gup_flags);
 
 	mmput(mm);
 
diff --git a/mm/nommu.c b/mm/nommu.c
index ed3934bc2de4..1fca0bb7f042 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1752,6 +1752,30 @@ static int __copy_remote_vm_str(struct mm_struct *mm, unsigned long addr,
 	return ret;
 }
 
+/**
+ * copy_remote_mm_str - copy a string from a remote address space.
+ * @mm:         the remote address space
+ * @addr:       start address to read from
+ * @buf:        destination buffer
+ * @len:        number of bytes to copy
+ * @gup_flags:  flags modifying lookup behaviour (unused)
+ *
+ * The caller must hold a reference on @mm.
+ *
+ * Return: number of bytes copied from @addr (source) to @buf (destination),
+ * not including the trailing NUL. If @len is zero, return 0 without accessing
+ * @buf. Otherwise, @buf is always NUL-terminated. On any error, return
+ * -EFAULT.
+ */
+int copy_remote_mm_str(struct mm_struct *mm, unsigned long addr,
+		       void *buf, int len, unsigned int gup_flags)
+{
+	if (unlikely(len == 0))
+		return 0;
+
+	return __copy_remote_vm_str(mm, addr, buf, len);
+}
+
 /**
  * copy_remote_vm_str - copy a string from another process's address space.
  * @tsk:	the task of the target address space
@@ -1781,7 +1805,7 @@ int copy_remote_vm_str(struct task_struct *tsk, unsigned long addr,
 		return -EFAULT;
 	}
 
-	ret = __copy_remote_vm_str(mm, addr, buf, len);
+	ret = copy_remote_mm_str(mm, addr, buf, len, gup_flags);
 
 	mmput(mm);
 
-- 
2.55.0