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

On Thu, May 20, 2021 at 4:11 AM Mark Rutland [off-list ref] wrote:

Hi,

On Thu, May 20, 2021 at 03:43:37AM -0700, Joe Richey wrote:

quoted

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.

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.

quoted

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).

That's a good idea. I mostly just did the open-coding for
consistency with the surrounding code, but aside from kvm.h
there aren't really many usages to change, so I can do it.

For kvm.h, I think it might be best to leave it open-coded
and move the entire file to _BITUL() at once.

quoted

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.

I'll just change the script to recommend _BITUL() (instead
of BIT()) if the code path contains "uapi".

Thanks,
Mark.

quoted

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