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

[Mark Rutland <mark.rutland@arm.com>]

Hi,

On Thu, May 20, 2021 at 03:43:37AM -0700, Joe Richey wrote:
> From: Joe Richey <joerichey@google.com>
> 
> 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. 

Beware that the common definition of BIT() (in include/vdso/bits.h) is:

| #define BIT(nr)                 (UL(1) << (nr))

That UL() can be important if `nr` is ever greater than bits per int.

> 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.

In include/uapi/linux/const.h we have an equivaleint _BITUL() macro,
which I think should be used in preference of open-coding this (and is
already used in a number of uapi headers).

> However, there really should be a check for this in checkpatch.pl
> Currently, the script actually _encourages_ users to use the BIT macro
> even if adding things to UAPI.

I think having something that suggests s/BIT()/_BITUL()/ under uapi
would be good.

Thanks,
Mark.

> 
> Running `rg "BIT\(" **/uapi/**` shows no more usage of BIT() in any
> UAPI headers. Tested by building a basic kernel. Changes are trivial.
> 
> Joe Richey (6):
>   x86/elf: Don't use BIT() macro in UAPI headers
>   KVM: X86: Don't use BIT() macro in UAPI headers
>   drivers: firmware: psci: Don't use BIT() macro in UAPI headers
>   uacce: Don't use BIT() macro in UAPI headers
>   media: vicodec: Don't use BIT() macro in UAPI headers
>   tools headers UAPI: Sync pkt_sched.h with the kernel sources
> 
>  arch/x86/include/uapi/asm/hwcap2.h   |   2 +-
>  include/uapi/linux/kvm.h             |   4 +-
>  include/uapi/linux/psci.h            |   2 +-
>  include/uapi/linux/v4l2-controls.h   |  22 ++---
>  include/uapi/misc/uacce/uacce.h      |   2 +-
>  tools/include/uapi/linux/kvm.h       |   4 +-
>  tools/include/uapi/linux/pkt_sched.h | 122 ++++++++++++++++++++++++---
>  7 files changed, 130 insertions(+), 28 deletions(-)
> 
> -- 
> 2.31.1
>