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From: Natanael Copa <ncopa@alpinelinux.org>
To: "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca>
Cc: musl@lists.openwall.com
Subject: Re: [musl] [PATCH] don't set errno in free
Date: Thu, 21 Jan 2021 16:50:00 +0100	[thread overview]
Message-ID: <20210121165000.61205767@ncopa-desktop.lan> (raw)
In-Reply-To: <20210121140240.83405-1-alex_y_xu@yahoo.ca>

On Thu, 21 Jan 2021 09:02:40 -0500
"Alex Xu (Hello71)" <alex_y_xu@yahoo.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

  reply	other threads:[~2021-01-21 15:50 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20210121140240.83405-1-alex_y_xu.ref@yahoo.ca>
2021-01-21 14:02 ` Alex Xu (Hello71)
2021-01-21 15:50   ` Natanael Copa [this message]
2021-01-21 16:18     ` Rich Felker
2021-01-21 16:20       ` Florian Weimer
2021-01-21 16:31       ` Natanael Copa
2021-01-21 16:27   ` Rich Felker

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