Re: [PATCH 2/8] log: Only check for FALLOC_FL_COLLAPSE_RANGE availability at runtime

[Stefano Brivio <sbrivio@redhat.com>]

On Wed,  6 Nov 2024 17:54:15 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> log.c has several #ifdefs on FALLOC_FL_COLLAPSE_RANGE that won't attempt
> to use it if not defined.  But even if the value is defined at compile
> time, it might not be available in the runtime kernel, so we need to check
> for errors from a fallocate() call and fall back to other methods.
> 
> Simplify this to only need the runtime check by using linux_dep.h to define
> FALLOC_FL_COLLAPSE_RANGE if it's not in the kernel headers.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> ---
>  Makefile    | 5 -----
>  linux_dep.h | 6 ++++++
>  log.c       | 9 +--------
>  3 files changed, 7 insertions(+), 13 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index 56bf2e8..cb91535 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -59,11 +59,6 @@ ifeq ($(shell :|$(CC) -fstack-protector-strong -S -xc - -o - >/dev/null 2>&1; ec
>  	FLAGS += -fstack-protector-strong
>  endif
>  
> -C := \#define _GNU_SOURCE\n\#include <fcntl.h>\nint x = FALLOC_FL_COLLAPSE_RANGE;
> -ifeq ($(shell printf "$(C)" | $(CC) -S -xc - -o - >/dev/null 2>&1; echo $$?),0)
> -	EXTRA_SYSCALLS += fallocate
> -endif
> -
>  prefix		?= /usr/local
>  exec_prefix	?= $(prefix)
>  bindir		?= $(exec_prefix)/bin
> diff --git a/linux_dep.h b/linux_dep.h
> index 8921623..eae9c3c 100644
> --- a/linux_dep.h
> +++ b/linux_dep.h
> @@ -119,4 +119,10 @@ struct tcp_info_linux {
>  						 */
>  };
>  
> +#include <linux/falloc.h>
> +
> +#ifndef FALLOC_FL_COLLAPSE_RANGE
> +#define FALLOC_FL_COLLAPSE_RANGE	0x08
> +#endif
> +
>  #endif /* LINUX_DEP_H */
> diff --git a/log.c b/log.c
> index 19f1d98..3c1b39c 100644
> --- a/log.c
> +++ b/log.c
> @@ -92,7 +92,6 @@ const char *logfile_prefix[] = {
>  	"         ",		/* LOG_DEBUG */
>  };
>  
> -#ifdef FALLOC_FL_COLLAPSE_RANGE

This breaks the build on Alpine (and I suppose on Void Linux too, that
is, whenever we build against musl):

log.c: In function 'logfile_rotate':
log.c:207:28: error: 'FALLOC_FL_COLLAPSE_RANGE' undeclared (first use in this function)
  207 |         if (!fallocate(fd, FALLOC_FL_COLLAPSE_RANGE, 0, log_cut_size))
      |                            ^~~~~~~~~~~~~~~~~~~~~~~~
log.c:207:28: note: each undeclared identifier is reported only once for each function it appears in

and it's fixed by including linux_dep.h from log.c.

-- 
Stefano