Re: [uml-devel] [PATCH] fix writing into /dev/kmem

[BlaisorBlade <blaisorblade_spam@yahoo.it>]

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Alle 03:36, domenica 29 febbraio 2004, Jeff Dike ha scritto:
> The patch below fixes a bug in the /dev/kmem driver which causes UML to
> die if you write to it.

Also, I forgot one thing, about this Oops:

the i386 Oopses, normally, kill only the thread which triggered the fault - 
while instead Uml panics altogether. Cannot this be changed at 
arch/um/kernel/trap_kern.c:segv()?

        if(!is_user)
-                panic("Kernel mode fault at addr 0x%lx, ip 0x%lx",
-                      address, ip);
+		 die("Oops".....) //some printk calls... can be omitted
+		 do_exit(SIGKILL);

like arch/i386/mm/fault.c: do_page_fault() ?
With the above change, cat /dev/kmem could only create an Oops on the cat 
process (and possibly some deadlock, but better than a straight crash).

By the way: in handle_page_fault(), there is some dead code (attached is the 
removal patch).

I also do not understand the loop around handle_mm_fault call, but I could be 
wrong: are you sure that after a successful handle_mm_fault (i.e. a major or 
minor fault) pte_present(*pte) could be false? If handle_mm_fault is not 
successful, we exit the loop anyway.

-- 
Paolo Giarrusso, aka Blaisorblade
Linux registered user n. 292729

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--- ./arch/um/kernel/trap_kern.c.fix	2004-02-20 18:43:26.000000000 +0100
+++ ./arch/um/kernel/trap_kern.c	2004-02-29 17:12:49.000000000 +0100
@@ -74,13 +74,7 @@
 			err = -ENOMEM;
 			goto out_of_memory;
 		default:
-			if (current->pid == 1) {
-				up_read(&mm->mmap_sem);
-				yield();
-				down_read(&mm->mmap_sem);
-				goto survive;
-			}
-			goto out;
+			BUG();
 		}
 		pte = pte_offset_kernel(pmd, page);
 	} while(!pte_present(*pte));