Subject: Re: [PATCH 0/6] Don't use BIT() macro in UAPI headers

From: Sean Christopherson <[email-masked]>
Date: Thu, 20 May 2021 15:47:46 +0000

On Thu, May 20, 2021, Joe Richey wrote:
> From: Joe Richey <[email-masked]>
> 
> The BIT(n) macro is used in the kernel as an alias for (1 << n).
> However, it is not defined in the UAPI headers, which means that any
> UAPI header files must be careful not to use it, or else the user
> will get a linker error. For example, compiling the following program:
> 
>     #include <sys/auxv.h>
>     #include <asm/hwcap2.h>
> 
>     // Detect if FSGSBASE instructions are enabled
>     int main() {
>         unsigned long val = getauxval(AT_HWCAP2);
>         return !(val & HWCAP2_FSGSBASE);
>     }
> 
> Results in the following likner error:
> 
>     /usr/bin/ld: /tmp/cceFpAdR.o: in function `main':
>     gs.c:(.text+0x21): undefined reference to `BIT'
> 
> This patch series changes all UAPI uses of BIT() to just be open-coded.
> However, there really should be a check for this in checkpatch.pl

Any reason not to provide such a patch in this series?  :-)

> Currently, the script actually _encourages_ users to use the BIT macro
> even if adding things to UAPI.
> 
> Running `rg "BIT\(" **/uapi/**` shows no more usage of BIT() in any
> UAPI headers. Tested by building a basic kernel. Changes are trivial.

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