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Date: Thu, 21 Jan 2021 16:50:00 +0100
From: Natanael Copa <ncopa@...inelinux.org>
To: "Alex Xu (Hello71)" <alex_y_xu@...oo.ca>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] don't set errno in free
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;
struct meta *g = get_meta(p);
int idx = get_slot_index(p);
size_t stride = get_stride(g);
@@ -133,11 +135,13 @@ void free(void *p)
g->freed_mask = freed+self;
else if (a_cas(&g->freed_mask, freed, freed+self)!=freed)
continue;
- return;
+ goto out;
}
wrlock();
struct mapinfo mi = nontrivial_free(g, idx);
unlock();
if (mi.len) munmap(mi.base, mi.len);
+out:
+ errno = orig_errno;
}
(looks like there are used names like errno_save, and old_errno in the code as well)
>
> // atomic free without locking if this is neither first or last slot
> @@ -139,5 +145,9 @@ void free(void *p)
> wrlock();
> struct mapinfo mi = nontrivial_free(g, idx);
> unlock();
> - if (mi.len) munmap(mi.base, mi.len);
> + // POSIX.1-202x requires free() to not modify errno on success
> + // munmap should succeed but no harm checking it again
> + if (mi.len)
> + if (munmap(mi.base, mi.len))
> + a_crash();
> }
This should go into separate commit.
-nc
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