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Date: Fri, 14 Feb 2020 23:37:04 -0300 From: Paul Cercueil <paul@...pouillou.net> To: Josh Poimboeuf <jpoimboe@...hat.com> Cc: Linus Walleij <linus.walleij@...aro.org>, linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>, Randy Dunlap <rdunlap@...radead.org> Subject: Re: [PATCH] pinctrl: ingenic: Make unreachable path more robust Le ven., févr. 14, 2020 at 14:37, Josh Poimboeuf <jpoimboe@...hat.com> a écrit : > On Fri, Feb 14, 2020 at 04:02:18PM -0300, Paul Cercueil wrote: >> Hi Josh, >> >> >> Le ven., févr. 14, 2020 at 10:37, Josh Poimboeuf >> <jpoimboe@...hat.com> a >> écrit : >> > In the second loop of ingenic_pinconf_set(), it annotates the >> switch >> > default case as unreachable(). The annotation is technically >> correct, >> > because that same case would have resulted in an early return in >> the >> > previous loop. >> > >> > However, if a bug were to get introduced later, for example if an >> > additional case were added to the first loop without adjusting the >> > second loop, it would result in nasty undefined behavior: most >> likely >> > the function's generated code would fall through to the next >> function. >> > >> > Another issue is that, while objtool normally understands >> unreachable() >> > annotations, there's one special case where it doesn't: when the >> > annotation occurs immediately after a 'ret' instruction. That >> happens >> > to be the case here because unreachable() is immediately before >> the >> > return. >> > >> > So change the unreachable() to BUG() so that the unreachable >> code, if >> > ever executed, would panic instead of introducing undefined >> behavior. >> > This also makes objtool happy. >> >> I don't like the idea that you change this driver's code just to >> work around >> a bug in objtool, and I don't like the idea of working around a >> future bug >> that shouldn't be introduced in the first place. > > It's not an objtool bug. It's a byproduct of the fact that GCC's > undefined behavior is inscrutable, and there's no way to determine > that > it actually *wants* to jump to a random function. > > And anyway, regardless of objtool, the patch is meant to make the code > more robust. > > Do you not agree that BUG (defined behavior) is more robust than > unreachable (undefined behavior)? It's a dead code path. That would be an undefined behaviour, if it was taken, but it's not. -Paul
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