Re: [PATCH 2/3] kcov: prefault the kcov_area

[Andrew Morton]

On Fri,  4 May 2018 14:55:34 +0100 Mark Rutland <mark.rutland@xxxxxxx> wrote:

> On many architectures the vmalloc area is lazily faulted in upon first
> access. This is problematic for KCOV, as __sanitizer_cov_trace_pc
> accesses the (vmalloc'd) kcov_area, and fault handling code may be
> instrumented. If an access to kcov_area faults, this will result in
> mutual recursion through the fault handling code and
> __sanitizer_cov_trace_pc(), eventually leading to stack corruption
> and/or overflow.
> 
> We can avoid this by faulting in the kcov_area before
> __sanitizer_cov_trace_pc() is permitted to access it. Once it has been
> faulted in, it will remain present in the process page tables, and will
> not fault again.
> 
> ...
>
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -324,6 +324,17 @@ static int kcov_close(struct inode *inode, struct file *filep)
>  	return 0;
>  }
>  
> +static void kcov_fault_in_area(struct kcov *kcov)

It would be nice to have a comment here explaining why the function
exists.

umm, this?

--- a/kernel/kcov.c~kcov-prefault-the-kcov_area-fix-fix
+++ a/kernel/kcov.c
@@ -324,6 +324,10 @@ static int kcov_close(struct inode *inod
 	return 0;
 }
 
+/*
+ * fault in a lazily-faulted vmalloc area, to avoid recursion issues if the
+ * vmalloc fault handler itself is instrumented.
+ */
 static void kcov_fault_in_area(struct kcov *kcov)
 {
 	unsigned long stride = PAGE_SIZE / sizeof(unsigned long);

> +{
> +	unsigned long stride = PAGE_SIZE / sizeof(unsigned long);
> +	unsigned long *area = kcov->area;
> +	unsigned long offset;
> +
> +	for (offset = 0; offset < kcov->size; offset += stride) {
> +		READ_ONCE(area[offset]);
> +	}
> +}