hurd: Fix mapping at address 0 with MAP_FIXED

Zero address passed to mmap () typically means the caller doesn't have
any specific preferred address. Not so if MAP_FIXED is passed: in this
case 0 means literal 0. Fix this case to pass anywhere = 0 into vm_map.

Also add some documentation.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
Message-Id: <20230625231751.404120-4-bugaevc@gmail.com>

diff --git a/sysdeps/mach/hurd/mmap.c b/sysdeps/mach/hurd/mmap.c
index 5aa7008393e1046efa169a4b7fdbec410bcd0eac..33672cf60ad6fbc5bb0a1e8050f7602bca6121cf 100644 (file)
--- a/sysdeps/mach/hurd/mmap.c
+++ b/sysdeps/mach/hurd/mmap.c
@@ -38,7 +38,7 @@ __mmap (void *addr, size_t len, int prot, int flags, int fd, off_t offset)
   vm_prot_t vmprot, max_vmprot;
   memory_object_t memobj;
   vm_address_t mapaddr, mask;
-  boolean_t copy;
+  boolean_t copy, anywhere;
 
   mapaddr = (vm_address_t) addr;
 
@@ -55,6 +55,7 @@ __mmap (void *addr, size_t len, int prot, int flags, int fd, off_t offset)
     vmprot |= VM_PROT_EXECUTE;
 
   copy = ! (flags & MAP_SHARED);
+  anywhere = ! (flags & MAP_FIXED);
 
 #ifdef __LP64__
   if ((addr == NULL) && (prot & PROT_EXEC)
@@ -141,9 +142,12 @@ __mmap (void *addr, size_t len, int prot, int flags, int fd, off_t offset)
   if (copy)
     max_vmprot = VM_PROT_ALL;
 
+  /* When ANYWHERE is true but the caller has provided a preferred address,
+     try mapping at that address with anywhere = 0 first.  If this fails,
+     we'll retry with anywhere = 1 below.  */
   err = __vm_map (__mach_task_self (),
                  &mapaddr, (vm_size_t) len, mask,
-                 mapaddr == 0,
+                 anywhere && (mapaddr == 0),
                  memobj, (vm_offset_t) offset,
                  copy, vmprot, max_vmprot,
                  copy ? VM_INHERIT_COPY : VM_INHERIT_SHARE);
@@ -165,7 +169,10 @@ __mmap (void *addr, size_t len, int prot, int flags, int fd, off_t offset)
     }
   else
     {
+      /* This mmap call is allowed to allocate anywhere,  */
       if (mapaddr != 0 && (err == KERN_NO_SPACE || err == KERN_INVALID_ADDRESS))
+        /* ...but above, we tried allocating at the specific address,
+           and failed to.  Now try again, with anywhere = 1 this time.  */
        err = __vm_map (__mach_task_self (),
                        &mapaddr, (vm_size_t) len, mask,
                        1, memobj, (vm_offset_t) offset,