Messages in this thread | | | Date | Wed, 22 Oct 2008 12:09:51 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 04/11] ftrace: comment arch ftrace code |
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On Wed, 22 Oct 2008 14:43:17 -0400 Steven Rostedt <rostedt@goodmis.org> wrote:
> Add comments to explain what is happening in the x86 arch ftrace code. > > Signed-off-by: Steven Rostedt <srostedt@redhat.com> > --- > arch/x86/kernel/ftrace.c | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > Index: linux-compile.git/arch/x86/kernel/ftrace.c > =================================================================== > --- linux-compile.git.orig/arch/x86/kernel/ftrace.c 2008-10-22 13:15:36.000000000 -0400 > +++ linux-compile.git/arch/x86/kernel/ftrace.c 2008-10-22 13:16:35.000000000 -0400 > @@ -66,18 +66,23 @@ ftrace_modify_code(unsigned long ip, uns > /* > * Note: Due to modules and __init, code can > * disappear and change, we need to protect against faulting > - * as well as code changing. > + * as well as code changing. We do this by using the > + * __copy_*_user functions. > * > * No real locking needed, this code is run through > * kstop_machine, or before SMP starts. > */ > + > + /* read the text we want to modify */ > if (__copy_from_user_inatomic(replaced, (char __user *)ip, > MCOUNT_INSN_SIZE)) > return -1; > > + /* Make sure it is what we expect it to be */ > if (memcmp(replaced, old_code, MCOUNT_INSN_SIZE) != 0) > return -1; > > + /* replace the text with the new text */ > if (__copy_to_user_inatomic((char __user *)ip, new_code, > MCOUNT_INSN_SIZE)) > return -1; >
I dunno.
__copy_to_user_inatomic() is for "copying memory from userspace while in an atomic context".
But what you're doing here is "modifying some kernel text which might generate a fault". It seems somewhat interface-abusive to use a userspace access function for that just because it happens right now to do the right thing.
I'd suggest that for clarity and for future-safety, you create some new interface function which does that thing. Right now it can be a simple wrapper around __copy_from_user_inatomic().
<looks>
oh, someone added one - probe_kernel_write(). Why not use that?
<wonders why he doesn't know what's going on any more>
Also, I hope that the above code is called from within a pagefault_disable()d region? Or are relying upon some magical side-effect of something which happens to do the same thing as pagefault_disable()? IOW: by what means does the above code ensure that do_page_fault() will see in_atomic()==true?
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