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

- From: Paolo Bonzini \<pbonzini@redhat.com\>
- Date: 2021-05-20 12:10:37 
- List: linux-arm-kernel
- Page: https://ratatoskr.run/linux-arm-kernel/2021/05/1385914
- Thread: https://ratatoskr.run/kvm/2021/05/1385907/t.md

```text
On 20/05/21 12:43, Joe Richey wrote:
> From: Joe Richey <redacted>
>
> 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
> 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.
>
> 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(-)
>

Will queue the KVM one, thanks!

Paolo


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