Re: Usage of strncpy in the kernel

On Wed, 8 Jan 2025, Martin Husemann wrote:

> Indeed, and also they can't be dead stores in the context this came up
> because we copyout() the result (or the (non-)initialized parts would not
> leak to userspace.
>

Right, but, I thought had drifted to discussing str{n,l}cpy()... :)

Anyway, just to show what modern compilers can do now:

DSE is not the only optimization that could happen. The compiler can do other
transformations. For example, substituting in-line, equivalent code, and so
eliminating calls to the standard library functions:

```
$ cc --version
cc (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
Copyright (C) 2023 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ cat s.c
#include <stdio.h>
#include <string.h>

int main(void) {
 	char buf[233];

 	strncpy(buf, "hello, world", sizeof buf);
 	printf("%s\n", buf);
 	return 0;
}
$ cc -O2 -march=native -S s.c 
$ egrep 'strncpy|printf' s.s
$
```

You can see what GCC's done here (this being almost a textbook optimization
case for it: small code; auto var.; constant string):

- Created a 256 byte stack.
- At compile-time, converted "hello, world" into a number (see hex).
- Copied this number onto the stack using AVX instructions.
- Zero-filled the rest of stack using the same.
and
- Substituted puts() for the printf() in the code.


We've seen GCC doing this sort of thing before on the list:
https://mail-index.netbsd.org/tech-userlevel/2024/09/15/msg014402.html

-RVP