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authorRich Felker <dalias@aerifal.cx>2014-08-16 02:41:45 -0400
committerRich Felker <dalias@aerifal.cx>2014-08-16 02:41:45 -0400
commit25d12fc0fc51f1fae0f85b4649a6463eb805aa8f (patch)
tree31307fc1e70d71c67592f5b405d0f01fce894713 /src/mman
parentb092f1c5fa9c048e12d002c7b972df5ecbe96d1d (diff)
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optimize locking against vm changes for mmap/munmap
the whole point of this locking is to prevent munmap, or mmap with MAP_FIXED, from deallocating virtual addresses, or changing the backing a given virtual address refers to, during certain race windows involving self-synchronized unmapping or destruction of pthread synchronization objects. there is no need for exclusion in the other direction, so it suffices to take the lock momentarily and release it before making the syscall, rather than holding it across the syscall.
Diffstat (limited to 'src/mman')
-rw-r--r--src/mman/mmap.c13
-rw-r--r--src/mman/munmap.c2
2 files changed, 7 insertions, 8 deletions
diff --git a/src/mman/mmap.c b/src/mman/mmap.c
index 1917a540..56e39a7a 100644
--- a/src/mman/mmap.c
+++ b/src/mman/mmap.c
@@ -16,8 +16,6 @@ weak_alias(dummy0, __vm_unlock);
void *__mmap(void *start, size_t len, int prot, int flags, int fd, off_t off)
{
- void *ret;
-
if (off & OFF_MASK) {
errno = EINVAL;
return MAP_FAILED;
@@ -26,14 +24,15 @@ void *__mmap(void *start, size_t len, int prot, int flags, int fd, off_t off)
errno = ENOMEM;
return MAP_FAILED;
}
- if (flags & MAP_FIXED) __vm_lock(-1);
+ if (flags & MAP_FIXED) {
+ __vm_lock(-1);
+ __vm_unlock();
+ }
#ifdef SYS_mmap2
- ret = (void *)syscall(SYS_mmap2, start, len, prot, flags, fd, off/UNIT);
+ return (void *)syscall(SYS_mmap2, start, len, prot, flags, fd, off/UNIT);
#else
- ret = (void *)syscall(SYS_mmap, start, len, prot, flags, fd, off);
+ return (void *)syscall(SYS_mmap, start, len, prot, flags, fd, off);
#endif
- if (flags & MAP_FIXED) __vm_unlock();
- return ret;
}
weak_alias(__mmap, mmap);
diff --git a/src/mman/munmap.c b/src/mman/munmap.c
index 8488d75c..359c691f 100644
--- a/src/mman/munmap.c
+++ b/src/mman/munmap.c
@@ -11,8 +11,8 @@ int __munmap(void *start, size_t len)
{
int ret;
__vm_lock(-1);
- ret = syscall(SYS_munmap, start, len);
__vm_unlock();
+ ret = syscall(SYS_munmap, start, len);
return ret;
}