Re: [PATCH v2] powerpc: Avoid code patching freed init sections

On 09/12/2018 05:36 PM, Michael Neuling wrote:

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--- a/arch/powerpc/lib/code-patching.c
+++ b/arch/powerpc/lib/code-patching.c

@@ -23,11 +23,33 @@
  #include <asm/code-patching.h>
  #include <asm/setup.h>
  
+

This blank line is not needed

Ack

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+static inline bool in_init_section(unsigned int *patch_addr)
+{
+	if (patch_addr < (unsigned int *)__init_begin)
+		return false;
+	if (patch_addr >= (unsigned int *)__init_end)
+		return false;
+	return true;
+}

Can we use the existing function init_section_contains() instead of this 
new function ?

Nice, I was looking for something like that... 

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+
+static inline bool init_freed(void)
+{
+	return (system_state >= SYSTEM_RUNNING);
+}
+

I would call this function differently, for instance init_is_finished(), 
because as you mentionned it doesn't exactly mean that init memory is freed.

Talking to Nick and mpe offline I think we are going to have to add a flag when
we free init mem rather than doing what we have now since what we have now has a
potential race. That change will eliminate the function entirely.

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  static int __patch_instruction(unsigned int *exec_addr, unsigned int
instr,
  			       unsigned int *patch_addr)
  {
  	int err;
  
+	/* Make sure we aren't patching a freed init section */
+	if (in_init_section(patch_addr) && init_freed()) {

The test must be done on exec_addr, not on patch_addr, as patch_addr is 
the address where the instruction as been remapped RW for allowing its 
modification.

Thanks for the catch

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Also I think it should be tested the other way round, because the 
init_freed() is a simpler test which will be false most of the time once 
the system is running so it should be checked first.

ok, I'll change.

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+		printk(KERN_DEBUG "Skipping init section patching addr:
0x%lx\n",

Maybe use pr_debug() instead.

Sure.

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+			(unsigned long)patch_addr);

Please align second line as per Codying style.

Sorry I can't see what's wrong. You're (or Cody :-P) going to have to spell it
this out for me...

I suspect that the suggestion is the opening parenthesis of "(unsigned long)" should sit directly under the "K" of "KERN_DEBUG". I'm pretty sure Documentation/process/coding-style.rst is very adamant that all identation is always 8 characters and spaces should never be used, but there still seems to be a lot of places/suggestions that argument lists that spill over multiple lines should be space indented to align with the very first argument at the top level. So, I guess I'm not sure what the desire is here. Although moving to pr_debug might fit it to a single line anyways. ;)

-Tyrel

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+		return 0;
+	}
+
  	__put_user_size(instr, patch_addr, 4, err);
  	if (err)
  		return err;

I think it would be better to put this verification in 
patch_instruction() instead, to avoid RW mapping/unmapping the 
instruction to patch when we are not going to do the patching.

If we do it there then we miss the raw_patch_intruction case.

IMHO I don't think we need to optimise this rare and non-critical path. 

Mikey