Follow @Openwall on Twitter for new release announcements and other news
[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Date: Sun, 11 Jun 2017 16:28:48 +0200
From: u-uy74@...ey.se
To: musl@...ts.openwall.com
Subject: Re: a workaround when mremap() is not functional?

On Sun, Jun 11, 2017 at 09:38:51AM -0400, Rich Felker wrote:
> > This was already on my radar because mremap for enlarging anon
> > mappings is also broken on nommu Linux. I think I just need to add a
> > "if failed, goto the existing malloc-and-memcpy-and-free code"
> > one-liner.
> 
> Does the attached work for you? It eliminates the old behavior of
> keeping the old mapping on failure when shrinking since that can lead
> to massive memory waste (e.g. malloc huge buffer in case you need it,
> then shrink to tiny one) if mremap failure-to-shrink is something that
> may be common and not just for the VMA-limit-exceeded case.

FreeBSD's linux_mremap() seems to be able to shrink the mapping
but it is not possible to know for all implementations.

> diff --git a/src/malloc/malloc.c b/src/malloc/malloc.c
> index c38c46f..d5ee428 100644
> --- a/src/malloc/malloc.c
> +++ b/src/malloc/malloc.c
> @@ -406,7 +406,7 @@ void *realloc(void *p, size_t n)
>  		if (oldlen == newlen) return p;
>  		base = __mremap(base, oldlen, newlen, MREMAP_MAYMOVE);
>  		if (base == (void *)-1)
> -			return newlen < oldlen ? p : 0;
> +			goto copy_realloc;
>  		self = (void *)(base + extra);
>  		self->csize = newlen - extra;
>  		return CHUNK_TO_MEM(self);
> @@ -439,6 +439,7 @@ void *realloc(void *p, size_t n)
>  		return CHUNK_TO_MEM(self);
>  	}
>  
> +copy_realloc:
>  	/* As a last resort, allocate a new chunk and copy to it. */
>  	new = malloc(n-OVERHEAD);
>  	if (!new) return 0;

This looks working on FreeBSD, as expected!

Thanks a lot for this very timely tweak, what a relief.

Regards,
Rune

Powered by blists - more mailing lists

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.