atags_proc: Add __counted_by for struct buffer and use struct_size()

Prepare for the coming implementation by GCC and Clang of the __counted_by attribute. Flexible array members annotated with __counted_by can have their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS (for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family functions).

While there, use struct_size() helper, instead of the open-coded version, to calculate the size for the allocation of the whole flexible structure, including of course, the flexible-array member.

This code was found with the help of Coccinelle, and audited and fixed manually.

Signed-off-by Gustavo A. R. Silva <[email protected]>

---
 arch/arm/kernel/atags_proc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

arch/arm/kernel/atags_proc.c +2 -3

index 3ec2afe78423..cd09f8ab93e3 100644
@@ -7,7 +7,7 @@
 
 struct buffer {
        size_t size;
-       char data[];
+       char data[] __counted_by(size);
 };
 
 static ssize_t atags_read(struct file *file, char __user *buf,

@@ -54,7 +54,7 @@ static int __init init_atags_procfs(void)
 
        WARN_ON(tag->hdr.tag != ATAG_NONE);
 
-       b = kmalloc(sizeof(*b) + size, GFP_KERNEL);
+       b = kmalloc(struct_size(b, data, size), GFP_KERNEL);
        if (!b)
                goto nomem;
 
--

2.34.1

Prepare for the coming implementation by GCC and Clang of the __counted_by attribute. Flexible array members annotated with __counted_by can have their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS (for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family functions).

While there, use struct_size() helper, instead of the open-coded version, to calculate the size for the allocation of the whole flexible structure, including of course, the flexible-array member.

This code was found with the help of Coccinelle, and audited and fixed manually.

Signed-off-by Gustavo A. R. Silva <[email protected]>

---
 arch/arm/kernel/atags_proc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

arch/arm/kernel/atags_proc.c +1 -1

index 3ec2afe78423..cd09f8ab93e3 100644
@@ -7,7 +7,7 @@
 
 struct buffer {
 	size_t size;
-	char data[];
+	char data[] __counted_by(size);
 };
 
 static ssize_t atags_read(struct file *file, char __user *buf,

Prepare for the coming implementation by GCC and Clang of the __counted_by attribute. Flexible array members annotated with __counted_by can have their accesses bounds-checked at run-time via CONFIG_UBSAN_BOUNDS (for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family functions).

While there, use struct_size() helper, instead of the open-coded version, to calculate the size for the allocation of the whole flexible structure, including of course, the flexible-array member.

[...]