[uml-devel] [patch] fix fault handling

[uml-devel] [patch] fix fault handling

[Gerd Knorr]

  Hi,

skas has a design bug: PTRACE_FAULTINFO doesn't pass enougth
informations.  Specifically the trap_no is missing, so you
don't know for sure whenever the SIGSEGV was a page fault or
a general protection fault.  I think that should be fixed in
skas4 (where we'll break backward compatibility anyway).

For the time being I've tried to fix that with the patch below,
which makes the uml kernel pass through general protection SEGV
to the app and seems to have no ill side effects.

  Gerd

--- linux-2.6.5/arch/um/kernel/skas/process.c.segv	2004-05-14 18:40:36.155621129 +0200
+++ linux-2.6.5/arch/um/kernel/skas/process.c	2004-05-14 19:49:00.762159068 +0200
@@ -54,6 +54,17 @@
 		panic("handle_segv - PTRACE_FAULTINFO failed, errno = %d\n",
 		      errno);
 
+	if (!(fault.is_write & 4)) {
+		/*
+		    kernel fault -- likely that was general
+ 		    protection fault (trap_no = 13), not a
+		    page fault (trap_no = 14).  But we don't
+		    know for sure as PTRACE_FAULTINFO doesn't
+		    tell us :-/
+		*/
+		bad_segv(fault.addr, 0, fault.is_write);
+		return;
+	}
 	segv(fault.addr, 0, FAULT_WRITE(fault.is_write), 1, NULL);
 }
 


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[uml-devel] Re: [patch] fix fault handling

[Jeff Dike]

kraxel@bytesex.org said:
> skas has a design bug: PTRACE_FAULTINFO doesn't pass enougth
> informations. 

Yeah, this is the biggest technical mistake in skas3.  I left it because I
didn't really want to touch the skas3 patch again, and instead fix it in
skas4.

				Jeff



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