[PATCH] NOMMU: Permit ptrace to ignore non-PROT_WRITE VMAs in NOMMU mode

Permit ptrace to modify a section that's non-shared but is marked
unwritable, such as is obtained by mapping the text segment of an ELF-FDPIC
executable binary with into a binary that's being ptraced[*].

[*] Under NOMMU conditions ptrace causes read-only MAP_PRIVATE mmaps to become
    totally private copies because if a private mapping was actually shared
    then the debugging setting breakpoints in it would potentially crash
    other processes.

This is done by using the VM_MAYWRITE flag rather than the VM_WRITE flag
when deciding whether to permit a write.

Without this patch a debugger can't set breakpoints in the mapped text
sections of executables that are mapped read-only private, even if the
mmap() syscall has taken a private copy because PT_PTRACED is set.

In addition, VM_MAYREAD is used instead of VM_READ for similar reasons.
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/nommu.c

@@ -1258,9 +1258,9 @@ int access_process_vm(struct task_struct *tsk, unsigned long addr, void *buf, in
 			len = vma->vm_end - addr;
 
 		/* only read or write mappings where it is permitted */
-		if (write && vma->vm_flags & VM_WRITE)
+		if (write && vma->vm_flags & VM_MAYWRITE)
 			len -= copy_to_user((void *) addr, buf, len);
-		else if (!write && vma->vm_flags & VM_READ)
+		else if (!write && vma->vm_flags & VM_MAYREAD)
 			len -= copy_from_user(buf, (void *) addr, len);
 		else
 			len = 0;