Messages in this thread | | | From | Nick Desaulniers <> | Date | Thu, 11 Aug 2022 11:39:15 -0700 | Subject | Re: mainline build failure for arm64 allmodconfig with clang |
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On Thu, Aug 11, 2022 at 8:39 AM Linus Torvalds <torvalds@linux-foundation.org> wrote: > > On Thu, Aug 11, 2022 at 8:05 AM Nathan Chancellor <nathan@kernel.org> wrote: > > > > Right, these are exposed by commit 258fafcd0683 ("Makefile.extrawarn: > > re-enable -Wformat for clang"). > > Christ. Why is clang's format warning SO COMPLETELY BROKEN? > > The warning is *WRONG*, for chrissake. Printing an 'int' with '%hhu' > is perfectly fine, and has well-defined semantics, and is what you > *want* to do in some cases.
Generally, printing an int with %hhu may truncate depending on the value of the int.
Perhaps there's something different we can be doing for literals though.
> I'm going to turn it off again, because honestly, this is a clang bug. > I don't care one whit if there are pending "fixes" for this clang bug, > until those fixes are in *clang*, not in the correct kernel code. > > For chrissake, the value it is trying to print out as a char is '3'.
If your referring to SOF_ABI_MAJOR from
commit b7bf23c0865f ("ASoC: SOF: ipc3-topology: Fix clang -Wformat warning")
in -next, 3 is an int literal. No truncation occurs, sure, but just use the correct format flag!
Otherwise please also considering reverting commit cbacb5ab0aa0 ("docs: printk-formats: Stop encouraging use of unnecessary %h[xudi] and %hh[xudi]") since for the past 3 years, we've been recommending that kernel developers not use %h or %hh. You allude to this in your "Admittedly, " note in commit 21f9c8a13bb2 ("Revert "Makefile.extrawarn: re-enable -Wformat for clang"") . Otherwise, please reinstate this patch.
I don't care which you pick, but let's be consistent?
Because having explicit documented practices then reverting things when those are followed is quite obnoxious.
> But even if it wasn't, and even if you wanted to print out "0xf365" as > a "char" value, then that is how C varargs *work*. It's an "int".
This is a different case than using a literal value in which no truncation would occur. (Your points about 3 and 'a' (no truncation) are distinct from 0xf365 (truncation)).
It would be anomolous to the compiler whether the truncation in such a case was intentional vs accidental.
printf("%hhx\n", 0xf365); // -Wformat: warning: format specifies type 'unsigned char' but the argument has type 'int'
should be
printf("%hhx\n", (unsigned char)0xf365); // intentional truncation, no warning
A cast in that case helps inform the compiler that "I know what I'm doing," and a comment helps code reviewers & maintainers.
> In fact, even a *character* is an "int". This program: > > #include <stdio.h> > > int main(int argc, char **argv) > { > printf("%hhu\n", 'a'); > } > > generates a warning with "clang -Wformat", and dammit, if you are a > clang developer and you see no problem with that warning, then I don't > know what to say.
Yeah, that is noisy. I think if we had an argument that is a literal, we should be able to tell then and there whether that value would result in truncation (and avoiding diagnosing if no truncation occurs, or split that into -Wformat-me-harder so that we could set -Wno-format-me-harder).
printf("%hhu\n", 256); // should this produce a warning? Which compilers do so? ;)
Though, isn't %c the correct format flag for characters?
> > Nathan, please make clang people see some sense. > > Because no, I'm not in the least interested in getting kernel "fixes" > for this issue. -Wformat for clang goes away until people have gotten > their heads extracted from their derrières. > > This is ridiculous. > > Linus
-- Thanks, ~Nick Desaulniers
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