Re: [PATCH] printf: fix Woverride-init warning for EDEADLK errno

[Rasmus Villemoes]

On 26/10/2020 22.49, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> On most architectures, gcc -Wextra warns about the list of error
> numbers containing both EDEADLK and EDEADLOCK:
> 
> lib/errname.c:15:67: warning: initialized field overwritten [-Woverride-init]
>    15 | #define E(err) [err + BUILD_BUG_ON_ZERO(err <= 0 || err > 300)] = "-" #err
>       |                                                                   ^~~
> lib/errname.c:172:2: note: in expansion of macro 'E'
>   172 |  E(EDEADLK), /* EDEADLOCK */
>       |  ^
> lib/errname.c:15:67: note: (near initialization for 'names_0[35]')
>    15 | #define E(err) [err + BUILD_BUG_ON_ZERO(err <= 0 || err > 300)] = "-" #err
>       |                                                                   ^~~
> lib/errname.c:172:2: note: in expansion of macro 'E'
>   172 |  E(EDEADLK), /* EDEADLOCK */
>       |  ^
> 
> Make that line conditional on the two values being distinct.
> 

NAK. That would end up using the "EDEADLOCK" string for the value 35 on
those architectures where they are the same, despite EDEADLK being the
by far the most used symbol. See the comments and original commit log,
the placement of these is deliberate.

How about we do this instead?

From: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>

The table of errno value->name contains a few duplicate entries since
e.g. EDEADLK == EDEADLOCK on most architectures. For the known cases,
the most used symbolic constant is listed last so that takes
precedence - the idea being that if someone sees "can't do that:
-EDEADLK" in dmesg, grepping for EDEADLK is more likely to find the
place where that error was generated (grepping for "can't do that"
will find the printk() that emitted it, but the source would often be
a few calls down).

However, that means one gets

  warning: initialized field overwritten [-Woverride-init]

when building with W=1. As the use of multiple initializers for the
same entry here is quite deliberate, explicitly disable that warning
for errname.o.

Reported-by: Arnd Bergmann <arnd@xxxxxxxxxx>
Fixes: 57f5677e535b ("printf: add support for printing symbolic error
names")
Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
---
 lib/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lib/Makefile b/lib/Makefile
index ce45af50983a2a5e3582..a98119519e100103818d 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -224,6 +224,7 @@ obj-$(CONFIG_HAVE_ARCH_TRACEHOOK) += syscall.o

 obj-$(CONFIG_DYNAMIC_DEBUG_CORE) += dynamic_debug.o
 obj-$(CONFIG_SYMBOLIC_ERRNAME) += errname.o
+CFLAGS_errname.o += $(call cc-disable-warning, override-init)

 obj-$(CONFIG_NLATTR) += nlattr.o

-- 
2.23.0