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Date: Thu, 21 Jan 2021 11:18:08 -0500
From: Rich Felker <dalias@...c.org>
To: Natanael Copa <ncopa@...inelinux.org>
Cc: "Alex Xu (Hello71)" <alex_y_xu@...oo.ca>, musl@...ts.openwall.com
Subject: Re: [PATCH] don't set errno in free
On Thu, Jan 21, 2021 at 04:50:00PM +0100, Natanael Copa wrote:
> On Thu, 21 Jan 2021 09:02:40 -0500
> "Alex Xu (Hello71)" <alex_y_xu@...oo.ca> wrote:
>
> > busybox echo fails if free sets errno, which madvise does on old
> > kernels.
> > ---
> > src/malloc/mallocng/free.c | 14 ++++++++++++--
> > 1 file changed, 12 insertions(+), 2 deletions(-)
> >
> > diff --git a/src/malloc/mallocng/free.c b/src/malloc/mallocng/free.c
> > index 40745f97..82836815 100644
> > --- a/src/malloc/mallocng/free.c
> > +++ b/src/malloc/mallocng/free.c
> > @@ -119,7 +119,13 @@ void free(void *p)
> > if (((uintptr_t)(start-1) ^ (uintptr_t)end) >= 2*PGSZ && g->last_idx) {
> > unsigned char *base = start + (-(uintptr_t)start & (PGSZ-1));
> > size_t len = (end-base) & -PGSZ;
> > - if (len) madvise(base, len, MADV_FREE);
> > + if (len) {
> > + // madvise(..., MADV_FREE) returns -EINVAL on old kernels
> > + // POSIX.1-202x requires free() to not modify errno on success
> > + int e = errno;
> > + madvise(base, len, MADV_FREE);
> > + errno = e;
> > + }
> > }
>
> I think we should save the errno early and make sure its restored on
> exit of the function. you should also include <errno.h>. I suggest
> something like:
>
> diff --git a/src/malloc/mallocng/free.c b/src/malloc/mallocng/free.c
> index 40745f97..77bed88b 100644
> --- a/src/malloc/mallocng/free.c
> +++ b/src/malloc/mallocng/free.c
> @@ -1,6 +1,7 @@
> #define _BSD_SOURCE
> #include <stdlib.h>
> #include <sys/mman.h>
> +#include <errno.h>
>
> #include "meta.h"
>
> @@ -102,6 +103,7 @@ void free(void *p)
> {
> if (!p) return;
>
> + int orig_errno = errno;
This is much costlier. It puts the TLS access (faulting and emulating
on old MIPS) in the path that runs on every call.
Rich
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