glibc and Linux-provided headers are known to generate macro
redefinition warnings when used together.  For example:
<linux/mman.h> and <sys/mman.h> duplicate some macro definitions.

We normally never see those warnings because GCC suppresses
warnings generated by system headers.  We carry our own copy of
Linux header files, though, and this makes those warnings not be
suppressed when glibc headers are included before Linux headers
(e.g. if <sys/mman.h> is included before <linux/mman.h>).

Use -isystem instead of -I for linux-headers.  This makes the
compiler treat our linux-headers directory the same way it treats
system-provided Linux headers, and suppress warnings generated by
them.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
Changes v1 -> v2:
* Use -isystem for $PWD/linux-headers too
  Reported-by: "Michael S. Tsirkin" <mst@redhat.com>
---
 Makefile.target | 2 +-
 configure       | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/Makefile.target b/Makefile.target
index 2d43dc586a..934a9f7431 100644
--- a/Makefile.target
+++ b/Makefile.target
@@ -12,7 +12,7 @@ endif
 
 $(call set-vpath, $(SRC_PATH):$(BUILD_DIR))
 ifdef CONFIG_LINUX
-QEMU_CFLAGS += -I../linux-headers
+QEMU_CFLAGS += -isystem ../linux-headers
 endif
 QEMU_CFLAGS += -iquote .. -iquote $(SRC_PATH)/target/$(TARGET_BASE_ARCH) -DNEED_CPU_H
 
diff --git a/configure b/configure
index cbf864bff1..bf5bf70600 100755
--- a/configure
+++ b/configure
@@ -899,7 +899,7 @@ Linux)
   linux="yes"
   linux_user="yes"
   kvm="yes"
-  QEMU_INCLUDES="-I\$(SRC_PATH)/linux-headers -I$PWD/linux-headers $QEMU_INCLUDES"
+  QEMU_INCLUDES="-isystem \$(SRC_PATH)/linux-headers -isystem $PWD/linux-headers $QEMU_INCLUDES"
   supported_os="yes"
   libudev="yes"
 ;;
-- 
2.24.1

On Wed, Mar 11, 2020 at 07:23:41PM -0400, Eduardo Habkost wrote:
> glibc and Linux-provided headers are known to generate macro
> redefinition warnings when used together.  For example:
> <linux/mman.h> and <sys/mman.h> duplicate some macro definitions.
> 
> We normally never see those warnings because GCC suppresses
> warnings generated by system headers.  We carry our own copy of
> Linux header files, though, and this makes those warnings not be
> suppressed when glibc headers are included before Linux headers
> (e.g. if <sys/mman.h> is included before <linux/mman.h>).
> 
> Use -isystem instead of -I for linux-headers.  This makes the
> compiler treat our linux-headers directory the same way it treats
> system-provided Linux headers, and suppress warnings generated by
> them.
> 
> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>

> ---
> Changes v1 -> v2:
> * Use -isystem for $PWD/linux-headers too
>   Reported-by: "Michael S. Tsirkin" <mst@redhat.com>
> ---
>  Makefile.target | 2 +-
>  configure       | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/Makefile.target b/Makefile.target
> index 2d43dc586a..934a9f7431 100644
> --- a/Makefile.target
> +++ b/Makefile.target
> @@ -12,7 +12,7 @@ endif
>  
>  $(call set-vpath, $(SRC_PATH):$(BUILD_DIR))
>  ifdef CONFIG_LINUX
> -QEMU_CFLAGS += -I../linux-headers
> +QEMU_CFLAGS += -isystem ../linux-headers
>  endif
>  QEMU_CFLAGS += -iquote .. -iquote $(SRC_PATH)/target/$(TARGET_BASE_ARCH) -DNEED_CPU_H
>  
> diff --git a/configure b/configure
> index cbf864bff1..bf5bf70600 100755
> --- a/configure
> +++ b/configure
> @@ -899,7 +899,7 @@ Linux)
>    linux="yes"
>    linux_user="yes"
>    kvm="yes"
> -  QEMU_INCLUDES="-I\$(SRC_PATH)/linux-headers -I$PWD/linux-headers $QEMU_INCLUDES"
> +  QEMU_INCLUDES="-isystem \$(SRC_PATH)/linux-headers -isystem $PWD/linux-headers $QEMU_INCLUDES"
>    supported_os="yes"
>    libudev="yes"
>  ;;
> -- 
> 2.24.1

On Wed, 11 Mar 2020 at 23:23, Eduardo Habkost <ehabkost@redhat.com> wrote:
>
> glibc and Linux-provided headers are known to generate macro
> redefinition warnings when used together.  For example:
> <linux/mman.h> and <sys/mman.h> duplicate some macro definitions.
>
> We normally never see those warnings because GCC suppresses
> warnings generated by system headers.  We carry our own copy of
> Linux header files, though, and this makes those warnings not be
> suppressed when glibc headers are included before Linux headers
> (e.g. if <sys/mman.h> is included before <linux/mman.h>).
>
> Use -isystem instead of -I for linux-headers.  This makes the
> compiler treat our linux-headers directory the same way it treats
> system-provided Linux headers, and suppress warnings generated by
> them.

I wondered whether this changed the search order for the headers
(ie whether it meant we now picked up our copy of the header
and not the system header in some cases), but both -I and -isystem
directories are searched before the standard system headers,
so the only effect is the warning suppression.

thanks
-- PMM