The UAPI references Kconfig's CONFIG_* macros (variables)

[Michael Witten <mfwitten@gmail.com>]

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The  UAPI  headers  contain  references  to  Kconfig's  `CONFIG_'
macros. Either this is a bug,  or there needs to be some standard
way for users  to provide definitions for these  macros, in order
to complete Linux's user-space API. Consider:

  #!/bin/sh
  cd "${linux_repo}"

  # Careful!
  #git reset --hard HEAD
  #git clean -fdx

  git checkout v5.1
  #zcat /proc/config.gz >.config

  mkdir -p /tmp/uapi/include
  make INSTALL_HDR_PATH=/tmp/uapi headers_install

  printf >/tmp/uapi/raw.c '%s\n%s\n' \
    '#include <linux/raw.h>' \
    'int main() { return MAX_RAW_MINORS; }'

Then:

  $ gcc -c -I/tmp/uapi/include /tmp/uapi/raw.c
  In file included from /tmp/uapi/raw.c:1:0:
  /tmp/uapi/raw.c: In function ‘main’:
  /tmp/uapi/include/linux/raw.h:17:24: error: ‘CONFIG_MAX_RAW_DEVS’ undeclared (first use in this function)
   #define MAX_RAW_MINORS CONFIG_MAX_RAW_DEVS
                          ^
  /tmp/uapi/raw.c:2:21: note: in expansion of macro ‘MAX_RAW_MINORS’
   int main() { return MAX_RAW_MINORS; }
                       ^~~~~~~~~~~~~~
  /tmp/uapi/include/linux/raw.h:17:24: note: each undeclared identifier is reported only once for each function it appears in
   #define MAX_RAW_MINORS CONFIG_MAX_RAW_DEVS
                          ^
  /tmp/uapi/raw.c:2:21: note: in expansion of macro ‘MAX_RAW_MINORS’
   int main() { return MAX_RAW_MINORS; }
                       ^~~~~~~~~~~~~~

As you can  see, the UAPI is actually incomplete;  there is not a
valid  definition for  `MAX_RAW_MINORS'.  Indeed,  on my  system,
`CONFIG_MAX_RAW_DEVS'  isn't   ever  defined   anywhere,  because
`CONFIG_RAW_DRIVER' is not set:

  $ git show v5.1:drivers/char/Kconfig | sed -n 467,469p
  config MAX_RAW_DEVS
          int "Maximum number of RAW devices to support (1-65536)"
          depends on RAW_DRIVER
  $ zcat /proc/config.gz | grep RAW_DRIVER
  # CONFIG_RAW_DRIVER is not set

Even if `CONFIG_RAW_DRIVER' were  set, the desired definition for
the  macro  `CONFIG_MAX_RAW_DEVS'  would  only be  found  in  the
following  header (generated  at  built-time),  which is  neither
officially nor likely available to user space:

  "${linux_repo}"/include/generated/autoconf.h

Other  such  references to  `CONFIG_*'  macros  are seen  in  the
following  (some  appear only  in  comments,  but perhaps  that's
conceptually a mistake, too):

  $ (cd /tmp/uapi/include && grep -R . -e \\bCONFIG_)
  ./asm/mman.h:#ifdef CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS
  ./asm/auxvec.h:#if defined(CONFIG_IA32_EMULATION) || !defined(CONFIG_X86_64)
  ./asm/e820.h: * kernel was built: MAX_NUMNODES == (1 << CONFIG_NODES_SHIFT),
  ./asm/e820.h: * unless the CONFIG_X86_PMEM_LEGACY option is set.
  ./asm/e820.h: * if CONFIG_INTEL_TXT is enabled, memory of this type will be
  ./mtd/ubi-user.h: * default kernel value of %CONFIG_MTD_UBI_BEB_LIMIT will be used.
  ./linux/pkt_cls.h:	TCA_FW_INDEV, /*  used by CONFIG_NET_CLS_IND */
  ./linux/pkt_cls.h:	TCA_FW_ACT, /* used by CONFIG_NET_CLS_ACT */
  ./linux/cm4000_cs.h: * member sizes. This leads to CONFIG_COMPAT breakage, since 32bit userspace
  ./linux/eventpoll.h:#ifdef CONFIG_PM_SLEEP
  ./linux/hw_breakpoint.h:#ifdef CONFIG_HAVE_MIXED_BREAKPOINTS_REGS
  ./linux/bpf.h: * has been built with CONFIG_EFFICIENT_UNALIGNED_ACCESS not set,
  ./linux/bpf.h: * 		the **CONFIG_CGROUP_NET_CLASSID** configuration option set to
  ./linux/bpf.h: * 		**CONFIG_IP_ROUTE_CLASSID** configuration option.
  ./linux/bpf.h: * 		with the **CONFIG_BPF_KPROBE_OVERRIDE** configuration
  ./linux/bpf.h: * 		the CONFIG_FUNCTION_ERROR_INJECTION option. As of this writing,
  ./linux/bpf.h: * 		**CONFIG_XFRM** configuration option.
  ./linux/bpf.h: *		the **CONFIG_BPF_LIRC_MODE2** configuration option set to
  ./linux/bpf.h: *		the **CONFIG_BPF_LIRC_MODE2** configuration option set to
  ./linux/bpf.h: * 		**CONFIG_SOCK_CGROUP_DATA** configuration option.
  ./linux/bpf.h: *		**CONFIG_NET** configuration option.
  ./linux/bpf.h: *		**CONFIG_NET** configuration option.
  ./linux/bpf.h: *		the **CONFIG_BPF_LIRC_MODE2** configuration option set to
  ./linux/raw.h:#define MAX_RAW_MINORS CONFIG_MAX_RAW_DEVS
  ./linux/pktcdvd.h:#if defined(CONFIG_CDROM_PKTCDVD_WCACHE)
  ./linux/flat.h:#ifdef CONFIG_BINFMT_SHARED_FLAT
  ./linux/videodev2.h: * Only implemented if CONFIG_VIDEO_ADV_DEBUG is defined.
  ./linux/elfcore.h:#ifdef CONFIG_BINFMT_ELF_FDPIC
  ./linux/atmdev.h:#ifdef CONFIG_COMPAT
  ./asm-generic/bitsperlong.h: * both 32 and 64 bit user space must not rely on CONFIG_64BIT
  ./asm-generic/unistd.h:/* mm/, CONFIG_MMU only */
  ./asm-generic/mman-common.h:#ifdef CONFIG_MMAP_ALLOW_UNINITIALIZED
  ./asm-generic/fcntl.h:#ifndef CONFIG_64BIT

What is the correct way to think about this?

  * Should the UAPI make no reference to build-time configurations?
  * Should the UAPI headers include sanity checks on behalf of the user?
  * Should there be a `/proc/config.h.gz' facility?

Sincerely,
Michael Witten