RE: [RFC PATCH v2 6/8] tools/nolibc/stdlib: Implement `malloc()`, `calloc()`, `realloc()` and `free()`

[David Laight <David.Laight@ACULAB.COM>]

From: Ammar Faizi
> Sent: 22 March 2022 10:21
> 
> Implement basic dynamic allocator functions. These functions are
> currently only available on architectures that have nolibc mmap()
> syscall implemented. These are not a super-fast memory allocator,
> but at least they can satisfy basic needs for having heap without
> libc.
> 
> Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
> ---
> 
> @@ Changelog:
> 
>    Link: https://lore.kernel.org/lkml/20220320093750.159991-6-ammarfaizi2@gnuweeb.org
>    RFC v1 -> RFC v2:
>     - Move container_of() and offsetof() macro to types.h with a
>       separate patch (comment from Willy).
> ---
>  tools/include/nolibc/stdlib.h | 68 +++++++++++++++++++++++++++++++++++
>  1 file changed, 68 insertions(+)
> 
> diff --git a/tools/include/nolibc/stdlib.h b/tools/include/nolibc/stdlib.h
> index aca8616335e3..a0ed75431e0a 100644
> --- a/tools/include/nolibc/stdlib.h
> +++ b/tools/include/nolibc/stdlib.h
> @@ -11,7 +11,12 @@
>  #include "arch.h"
>  #include "types.h"
>  #include "sys.h"
> +#include "string.h"
> 
> +struct nolibc_heap {
> +	size_t	len;
> +	char	user_p[] __attribute__((__aligned__));

Doesn't that need (number) in the attribute?

> +};
> 
>  /* Buffer used to store int-to-ASCII conversions. Will only be implemented if
>   * any of the related functions is implemented. The area is large enough to
> @@ -60,6 +65,18 @@ int atoi(const char *s)
>  	return atol(s);
>  }
> 
> +static __attribute__((unused))
> +void free(void *ptr)
> +{
> +	struct nolibc_heap *heap;
> +
> +	if (!ptr)
> +		return;
> +
> +	heap = container_of(ptr, struct nolibc_heap, user_p);
> +	munmap(heap, heap->len);
> +}
> +
>  /* Tries to find the environment variable named <name> in the environment array
>   * pointed to by global variable "environ" which must be declared as a char **,
>   * and must be terminated by a NULL (it is recommended to set this variable to
> @@ -205,6 +222,57 @@ char *ltoa(long in)
>  	return itoa_buffer;
>  }
> 
> +static __attribute__((unused))
> +void *malloc(size_t len)
> +{
> +	struct nolibc_heap *heap;

If you do (say):
	len = ROUNDUP(len + sizeof *heap, 4096)
you can optimise a lot of the realloc() calls.

I actually wonder if compiling a mini-libc.a
and then linking the programs against it might
be better than all these static functions?
-ffunction-sections can help a bit (where supported).

	David
	
> +
> +	heap = mmap(NULL, sizeof(*heap) + len, PROT_READ|PROT_WRITE,
> +		    MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
> +	if (__builtin_expect(heap == MAP_FAILED, 0))
> +		return NULL;
> +
> +	heap->len = sizeof(*heap) + len;
> +	return heap->user_p;
> +}
> +
> +static __attribute__((unused))
> +void *calloc(size_t size, size_t nmemb)
> +{
> +	void *orig;
> +	size_t res = 0;
> +
> +	if (__builtin_expect(__builtin_mul_overflow(nmemb, size, &res), 0)) {
> +		SET_ERRNO(ENOMEM);
> +		return NULL;
> +	}
> +
> +	/*
> +	 * No need to zero the heap, the MAP_ANONYMOUS in malloc()
> +	 * already does it.
> +	 */
> +	return malloc(res);
> +}
> +
> +static __attribute__((unused))
> +void *realloc(void *old_ptr, size_t new_size)
> +{
> +	struct nolibc_heap *heap;
> +	void *ret;
> +
> +	ret = malloc(new_size);
> +	if (__builtin_expect(!ret, 0))
> +		return NULL;
> +
> +	if (!old_ptr)
> +		return ret;
> +
> +	heap = container_of(old_ptr, struct nolibc_heap, user_p);
> +	memcpy(ret, heap->user_p, heap->len);
> +	munmap(heap, heap->len);
> +	return ret;
> +}
> +
>  /* converts unsigned long integer <in> to a string using the static itoa_buffer
>   * and returns the pointer to that string.
>   */
> --
> Ammar Faizi

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