On Mon, Nov 03, 2025 at 07:29:08PM +0000, David Laight wrote: > On Mon, 3 Nov 2025 20:16:46 +0200 > Andy Shevchenko <[email protected]> wrote: > > On Mon, Nov 03, 2025 at 04:41:41PM +0200, Andy Shevchenko wrote: > > > On Mon, Nov 03, 2025 at 01:22:13PM +0000, David Laight wrote:
... > > > Pragma will be hated. > > They have been used in a few other places. > and to disable more 'useful' warnings. You can go with pragma, but even though it just hides the potential issues. Not my choice. > > > I believe there is a better way to do what you want. Let me cook a PoC. > > > > I tried locally several approaches and the best I can come up with is the > > pre-generated > > (via Python script) pieces of C code that we can copy'n'paste instead of > > that shortened > > form. So basically having a full 256 tables in the code is my suggestion to > > fix the build > > issue. Alternatively we can generate that at run-time (on the first run) in > > the similar way how prime_numbers.c does. The downside of such an approach > > is loosing > > the const specifier, which I consider kinda important. > > > > Btw, in the future here might be also the side-channel attack concerns > > appear, which would > > require to reconsider the whole algo to get it constant-time execution. > > The array lookup version is 'reasonably' time constant. The array doesn't fit the cacheline. > One option is to offset all the array entries by 1 and subtract 1 after > reading the entry. Yes, I was thinking of it, but found a bit weird. > That means that the 'error' characters have zero in the array (not -1). > At least the compiler won't error that! > The extra 'subtract 1' is probably just measurable. > But I'd consider raising a bug on gcc :-) And clang? :-) > One of the uses of ranged designated initialisers for arrays is to change the > default value - as been done here. > It shouldn't cause a warning. This is prone to mistakes when it's not the default rewrite. I fixed already twice such an issue in drivers/hid in the past few months. -- With Best Regards, Andy Shevchenko
