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Message-Id: <20170628165243.16502-1-amonakov@ispras.ru>
Date: Wed, 28 Jun 2017 19:52:43 +0300
From: Alexander Monakov <amonakov@...ras.ru>
To: musl@...ts.openwall.com
Cc: Timo Teräs <timo.teras@....fi>
Subject: [RFC PATCH] reduce severity of ldso reclaim_gaps hack
---
In part inspired by an earlier patch by Timo Teräs.
Without --gc-sections, __malloc_donate is dead weight in static linking. This
can be solved by moving it to a separate translation unit (but for that matter,
realloc is much heavier and can be factored out too).
Extra jumps from 'free' to 'bin_chunk' and their non-contiguous code layout is
admittedly a problem, but I believe the impact is miniscule. As a minor
consolation, entries to bin_chunk from malloc now require fewer branches, and
hopefully new code is easier to follow.
Alexander
ldso/dynlink.c | 15 +++--------
src/malloc/malloc.c | 71 ++++++++++++++++++++++++++++++++++++-----------------
2 files changed, 52 insertions(+), 34 deletions(-)
diff --git a/ldso/dynlink.c b/ldso/dynlink.c
index d20dbd87..985592ce 100644
--- a/ldso/dynlink.c
+++ b/ldso/dynlink.c
@@ -475,23 +475,14 @@ 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 ef4c7368..b56fdaa2 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)
@@ -410,10 +412,9 @@ void *realloc(void *p, size_t n)
size_t newlen = n + extra;
/* Crash on realloc of freed chunk */
if (extra & 1) a_crash();
- if (newlen < PAGE_SIZE && (new = malloc(n))) {
- memcpy(new, p, n-OVERHEAD);
- free(p);
- return new;
+ if (newlen < PAGE_SIZE && (new = malloc(n-OVERHEAD))) {
+ n0 = n;
+ goto copy_free_ret;
}
newlen = (newlen + PAGE_SIZE-1) & -PAGE_SIZE;
if (oldlen == newlen) return p;
@@ -456,34 +457,20 @@ copy_realloc:
/* As a last resort, allocate a new chunk and copy to it. */
new = malloc(n-OVERHEAD);
if (!new) return 0;
+copy_free_ret:
memcpy(new, p, n0-OVERHEAD);
free(CHUNK_TO_MEM(self));
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();
@@ -544,3 +531,43 @@ void free(void *p)
unlock_bin(i);
}
+
+#if defined(__GNUC__)
+__attribute__((cold))
+#endif
+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)
+{
+ if (end - start < 2*OVERHEAD + SIZE_ALIGN) return;
+ start += OVERHEAD + SIZE_ALIGN - 1;
+ start -= (uintptr_t)start & (SIZE_ALIGN - 1);
+ end -= (uintptr_t)end & (SIZE_ALIGN - 1);
+ if (end - start < OVERHEAD + SIZE_ALIGN) return;
+ if (end - start >= MMAP_THRESHOLD) return;
+
+ struct chunk *c = MEM_TO_CHUNK(start), *n = MEM_TO_CHUNK(end);
+ c->psize = n->csize = C_INUSE;
+ c->csize = n->psize = (end - start) | C_INUSE;
+ bin_chunk(c);
+}
--
2.11.0
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