PATCH: Make libmmalloc work properly with /dev/zero
Scott Pakin
pakin@uiuc.edu
Tue Mar 5 14:11:00 GMT 2002
I just tried using libmmalloc for the first time, and I was unable to
get it to work when allocating memory from /dev/zero (e.g., by using
"mmalloc_attach (-1, NULL)" to mmap() it). libmmalloc bus errors when
it tries to write to the allocated region. The library works fine
when I use an ordinary file, however.
I'm using the libmmalloc.a that comes with gdb-5.1, and I'm running
under Linux 2.4.2. /dev/zero has 0666 permissions.
I believe the bug is that libmmalloc always mmap()s files with
MAP_SHARED, but it ought to map /dev/zero with MAP_PRIVATE, because
/dev/zero really isn't writable. Attached below is a patch to
mmap-sup.c that makes mmalloc conditionally use one of MAP_SHARED or
MAP_PRIVATE, based on whether or not the library explicitly opened
/dev/zero.
Of course, if there's some subtle reason the code was written the way
it was -- or if I'm doing something egregiously wrong -- please let me
know.
-- Scott
================== diff -Naur mmap-sup.c.ORIG mmap-sup.c ==================
--- mmap-sup.c.ORIG Tue Mar 5 13:53:57 2002
+++ mmap-sup.c Tue Mar 5 14:12:16 2002
@@ -48,6 +48,15 @@
#define PAGE_ALIGN(addr) (caddr_t) (((long)(addr) + pagesize - 1) & \
~(pagesize - 1))
+
+/* Return MAP_PRIVATE if MDP represents /dev/zero. Otherwise, return
+ MAP_SHARED. */
+
+#define MAP_PRIVATE_OR_SHARED(MDP) ((MDP -> flags & MMALLOC_DEVZERO) ? \
+
MAP_PRIVATE : \
+
MAP_SHARED)
+
+
/* Get core for the memory region specified by MDP, using SIZE as the
amount to either add to or subtract from the existing region. Works
like sbrk(), but using mmap(). */
@@ -113,7 +122,7 @@
{
/* Let mmap pick the map start address */
mapto = mmap (0, mapbytes, PROT_READ | PROT_WRITE,
-
MAP_SHARED, mdp -> fd, foffset);
+
MAP_PRIVATE_OR_SHARED(mdp), mdp -> fd, foffset);
if (mapto != (caddr_t) -1)
{
mdp -> base = mdp -> breakval = mapto;
@@ -125,7 +134,8 @@
else
{
mapto = mmap (mdp -> top, mapbytes, PROT_READ | PROT_WRITE,
-
MAP_SHARED | MAP_FIXED, mdp -> fd, foffset);
+
MAP_PRIVATE_OR_SHARED(mdp) | MAP_FIXED, mdp -> fd,
+
foffset);
if (mapto == mdp -> top)
{
mdp -> top = moveto;
@@ -152,7 +162,7 @@
/* FIXME: Quick hack, needs error checking and other attention. */
base = mmap (mdp -> base, mdp -> top - mdp -> base,
-
PROT_READ | PROT_WRITE, MAP_SHARED | MAP_FIXED,
+
PROT_READ | PROT_WRITE, MAP_PRIVATE_OR_SHARED(mdp) | MAP_FIXED,
mdp -> fd, 0);
return ((PTR) base);
}
@@ -166,13 +176,13 @@
caddr_t base = NULL;
#ifdef MAP_ANONYMOUS
- flags = MAP_SHARED | MAP_ANONYMOUS;
+ flags = MAP_PRIVATE | MAP_ANONYMOUS;
fd = -1;
#else
#ifdef MAP_FILE
- flags = MAP_SHARED | MAP_FILE;
+ flags = MAP_PRIVATE | MAP_FILE;
#else
- flags = MAP_SHARED;
+ flags = MAP_PRIVATE;
#endif
fd = open ("/dev/zero", O_RDWR);
if (fd != -1)
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