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Date: Mon, 16 Apr 2018 20:54:36 +0300
From: Alexander Monakov <amonakov@...ras.ru>
To: musl@...ts.openwall.com
Subject: [PATCH 2/2] ldso, malloc: implement reclaim_gaps via __malloc_donate

Split 'free' into unmap_chunk and bin_chunk, use the latter to introduce
__malloc_donate and use it in reclaim_gaps instead of calling 'free'.
---
v2:
* drop attribute-cold annotation
* adjust __malloc_donate:
	 * drop comparison against MMAP_THRESHOLD
	 * check donated chunk size just once

 ldso/dynlink.c      | 15 +++-----------
 src/malloc/malloc.c | 60 +++++++++++++++++++++++++++++++++++++----------------
 2 files changed, 45 insertions(+), 30 deletions(-)

diff --git a/ldso/dynlink.c b/ldso/dynlink.c
index 9bf6924b..bdf901dd 100644
--- a/ldso/dynlink.c
+++ b/ldso/dynlink.c
@@ -476,23 +476,14 @@ static void redo_lazy_relocs()
 /* A huge hack: to make up for the wastefulness of shared libraries
  * needing at least a page of dirty memory even if they have no global
  * data, we reclaim the gaps at the beginning and end of writable maps
- * and "donate" them to the heap by setting up minimal malloc
- * structures and then freeing them. */
+ * and "donate" them to the heap. */
 
 static void reclaim(struct dso *dso, size_t start, size_t end)
 {
-	size_t *a, *z;
+	void __malloc_donate(char *, char *);
 	if (start >= dso->relro_start && start < dso->relro_end) start = dso->relro_end;
 	if (end   >= dso->relro_start && end   < dso->relro_end) end = dso->relro_start;
-	start = start + 6*sizeof(size_t)-1 & -4*sizeof(size_t);
-	end = (end & -4*sizeof(size_t)) - 2*sizeof(size_t);
-	if (start>end || end-start < 4*sizeof(size_t)) return;
-	a = laddr(dso, start);
-	z = laddr(dso, end);
-	a[-2] = 1;
-	a[-1] = z[0] = end-start + 2*sizeof(size_t) | 1;
-	z[1] = 1;
-	free(a);
+	__malloc_donate(laddr(dso, start), laddr(dso, end));
 }
 
 static void reclaim_gaps(struct dso *dso)
diff --git a/src/malloc/malloc.c b/src/malloc/malloc.c
index 1af4ae5a..2666fe1b 100644
--- a/src/malloc/malloc.c
+++ b/src/malloc/malloc.c
@@ -299,6 +299,8 @@ static int pretrim(struct chunk *self, size_t n, int i, int j)
 	return 1;
 }
 
+static void bin_chunk(struct chunk *);
+
 static void trim(struct chunk *self, size_t n)
 {
 	size_t n1 = CHUNK_SIZE(self);
@@ -314,7 +316,7 @@ static void trim(struct chunk *self, size_t n)
 	next->psize = n1-n | C_INUSE;
 	self->csize = n | C_INUSE;
 
-	free(CHUNK_TO_MEM(split));
+	bin_chunk(split);
 }
 
 void *malloc(size_t n)
@@ -448,29 +450,14 @@ copy_free_ret:
 	return new;
 }
 
-void free(void *p)
+static void bin_chunk(struct chunk *self)
 {
-	struct chunk *self, *next;
+	struct chunk *next = NEXT_CHUNK(self);
 	size_t final_size, new_size, size;
 	int reclaim=0;
 	int i;
 
-	if (!p) return;
-
-	self = MEM_TO_CHUNK(p);
-
-	if (IS_MMAPPED(self)) {
-		size_t extra = self->psize;
-		char *base = (char *)self - extra;
-		size_t len = CHUNK_SIZE(self) + extra;
-		/* Crash on double free */
-		if (extra & 1) a_crash();
-		__munmap(base, len);
-		return;
-	}
-
 	final_size = new_size = CHUNK_SIZE(self);
-	next = NEXT_CHUNK(self);
 
 	/* Crash on corrupted footer (likely from buffer overflow) */
 	if (next->psize != self->csize) a_crash();
@@ -531,3 +518,40 @@ void free(void *p)
 
 	unlock_bin(i);
 }
+
+static void unmap_chunk(struct chunk *self)
+{
+	size_t extra = self->psize;
+	char *base = (char *)self - extra;
+	size_t len = CHUNK_SIZE(self) + extra;
+	/* Crash on double free */
+	if (extra & 1) a_crash();
+	__munmap(base, len);
+}
+
+void free(void *p)
+{
+	if (!p) return;
+
+	struct chunk *self = MEM_TO_CHUNK(p);
+
+	if (IS_MMAPPED(self))
+		unmap_chunk(self);
+	else
+		bin_chunk(self);
+}
+
+void __malloc_donate(char *start, char *end)
+{
+	ssize_t align_start_up = (SIZE_ALIGN - 1) & -(uintptr_t)start;
+	ssize_t align_end_down = (SIZE_ALIGN - 1) &  (uintptr_t)end;
+	ssize_t chunk_size = end - start - (OVERHEAD + align_start_up + align_end_down);
+	if (chunk_size < OVERHEAD + SIZE_ALIGN) return;
+	start += align_start_up;
+	end   -= align_end_down;
+
+	struct chunk *c = MEM_TO_CHUNK(start + OVERHEAD), *n = MEM_TO_CHUNK(end);
+	c->psize = n->csize = C_INUSE;
+	c->csize = n->psize = C_INUSE | chunk_size;
+	bin_chunk(c);
+}
-- 
2.11.0

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