On Thu, 2020-08-27 at 15:48 +0100, Alex Dewar wrote: > On Thu, Aug 27, 2020 at 03:41:06PM +0200, Rasmus Villemoes wrote: > > On 27/08/2020 15.18, Alex Dewar wrote: > > > On Thu, Aug 27, 2020 at 09:15:37AM +0200, Greg Kroah-Hartman wrote: > > > > On Thu, Aug 27, 2020 at 08:42:06AM +0200, Rasmus Villemoes wrote: > > > > > On 25/08/2020 00.23, Alex Dewar wrote: > > > > > > kernel/cpu.c: don't use snprintf() for sysfs attrs > > > > > > > > > > > > As per the documentation (Documentation/filesystems/sysfs.rst), > > > > > > snprintf() should not be used for formatting values returned by sysfs. > > > > > > > > > > > > > > > > Can we have a sysfs_sprintf() (could just be a macro that does sprintf) > > > > > to make it clear to the next reader that we know we're in a sysfs show > > > > > method? It would make auditing uses of sprintf() much easier. > > > > > > > > Code churn to keep code checkers quiet for pointless reasons? What > > > > could go wrong with that... > > > > I did not (mean to) suggest replacing existing sprintf() calls in sysfs > > show methods. But when changes _are_ being made, such as when replacing > > snprintf() calls for whatever reasons, can we please not make it harder > > for people doing manual audits (those are "code checkers" as well, I > > suppose, but they do tend to only make noise when finding something). > > > > > > It should be pretty obvious to any reader that you are in a sysfs show > > > > method, as almost all of them are trivially tiny and obvious. > > > > git grep doesn't immediately show that, not even with a suitable -C > > argument, as you can't really know the potential callers unless you open > > the file and see that the function is only assigned as a .show method. > > And even that can be a pain because it's all hidden behind five levels > > of magic macros that build identifiers with ##. > > > > > Perhaps I should have mentioned this in the commit message, but the problem > > > is that snprintf() doesn't return the number of bytes written to the > > > destination buffer, > > > > I'm perfectly well aware of that, TYVM (you may want to 'git log > > --author Villemoes lib/vsprintf.c'). > > > > but the number of bytes that *would have been written if > > > they fitted*, which may be more than the bounds specified [1]. So "return > > > snprintf(...)" for sysfs attributes is an antipattern. If you need bounded > > > string ops, scnprintf() is the way to go. Using snprintf() can give a > > > false sense of security, because it isn't necessarily safe. > > > > Huh? This all seems utterly irrelevant WRT a change that replaces > > PAGE_SIZE by INT_MAX (because that's what sprintf() is going to pretend > > you passed). You get the same return value. > > > > But I'm not at all concerned about whether one passes the proper buffer > > size or not in sysfs show methods; with my embedded hat on, I'm all for > > saving a few bytes of .text here and there. The problem, as far as I'm > > concerned, is merely that adding sprintf() callers makes it harder to > > find the problematic sprintf() instances. > > > > Apologies, I think I might have expressed myself poorly, being a kernel noob > ;-). I know that this is a stylistic change rather than a functional > one -- I meant that I was hoping that it would be helpful to get rid of bad > uses of snprintf(). > > I really like your idea of helper methods though :-). If in show() > methods we could have something like: > return sysfs_itoa(buf, i); > in place of: > return sprintf(buf, "%d\n", i); > > ... then we wouldn't be introducing any new calls to sprintf() as you > say, but we'd still be removing a call to snprintf() (which also may be > problematic). Plus we'd have type checking on the argument. > > For returning strings, we could have a bounded and unbounded variant of > the function. As it seems like only single values should be returned via > sysfs, if we did things this way then it would only be these > string-returning functions which could cause buffer overflow problems > and kernel devs could focus their attention accordingly... > > What do people think? I'm happy to have a crack, provided this is > actually a sensible thing to do! I'm looking for a newbie-level project > to get started with. Not a bad idea. Coccinelle should be able to transform the various .show methods to something sysfs_ prefixed in a fairly automated way.
