Re: [Xen-devel] [PATCH v2] tools/Rules.mk: Don't optimize debug builds; add macro debugging information

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2015-01-13 at 19:17 +0800, Wen Congyang wrote:
> On 01/13/2015 06:11 PM, Ian Campbell wrote:
> > On Tue, 2015-01-13 at 13:52 +0800, Wen Congyang wrote:
> >> On 12/01/2014 10:21 PM, Euan Harris wrote:
> >>> Tools debug builds are built with optimization level -O1, inherited from
> >>> the CFLAGS definition in StdGNU.mk.   Optimizations confuse the debugger,
> >>> and the comment justifying -O1 in StdGNU.mk should not apply for a
> >>> userspace library.   Disable optimization by appending -O0 to CFLAGS,
> >>> which overrides the -O1 flag specified earlier.
> >>>
> >>> Also specify -g3, to add macro debugging information which allows
> >>> gdb to expand macro invocations.   This is useful as libxl uses many
> >>> non-trivial macros.
> >>>
> >>> Signed-off-by: Euan Harris <euan.harris@xxxxxxxxxx>
> >>>
> >>> Changes since v1:
> >>>   * moved flag override to tools/Rules.mk so it affects all tools
> >>> ---
> >>>  tools/Rules.mk |    5 +++++
> >>>  1 files changed, 5 insertions(+), 0 deletions(-)
> >>>
> >>> diff --git a/tools/Rules.mk b/tools/Rules.mk
> >>> index 87a56dc..7ef1ce5 100644
> >>> --- a/tools/Rules.mk
> >>> +++ b/tools/Rules.mk
> >>> @@ -54,6 +54,11 @@ CFLAGS_libxenvchan = -I$(XEN_LIBVCHAN)
> >>>  LDLIBS_libxenvchan = $(SHLIB_libxenctrl) $(SHLIB_libxenstore) 
> >>> -L$(XEN_LIBVCHAN) -lxenvchan
> >>>  SHLIB_libxenvchan  = -Wl,-rpath-link=$(XEN_LIBVCHAN)
> >>>  
> >>> +ifeq ($(debug),y)
> >>> +# Disable optimizations and debugging information for macros
> >>> +CFLAGS += -O0 -g3
> >>> +endif
> >>> +
> >>>  LIBXL_BLKTAP ?= $(CONFIG_BLKTAP2)
> >>>  
> >>>  ifeq ($(LIBXL_BLKTAP),y)
> >>>
> >>
> >> This patch causes a building error:
> >> gcc -fno-strict-aliasing -O2 -g -pipe -Wall -Werror=format-security 
> >> -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong 
> >> --param=ssp-buffer-size=4 -grecord-gcc-switches -m64 -mtune=generic 
> >> -D_GNU_SOURCE -fPIC -fwrapv -DNDEBUG -O2 -g -pipe -Wall 
> >> -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -fexceptions 
> >> -fstack-protector-strong --param=ssp-buffer-size=4 -grecord-gcc-switches 
> >> -m64 -mtune=generic -D_GNU_SOURCE -fPIC -fwrapv -O1 
> >> -fno-omit-frame-pointer -m64 -g -fno-strict-aliasing -std=gnu99 -Wall 
> >> -Wstrict-prototypes -Wdeclaration-after-statement 
> >> -Wno-unused-but-set-variable -Wno-unused-local-typedefs -O0 -g3 
> >> -D__XEN_TOOLS__ -MMD -MF .install.d -D_LARGEFILE_SOURCE 
> >> -D_LARGEFILE64_SOURCE -fno-optimize-sibling-calls -fPIC 
> >> -I../../tools/include -I../../tools/libxc/include -Ixen/lowlevel/xc 
> >> -I/usr/include/python2.7 -c xen/lowlevel/xc/xc.c -o 
> >> build/temp.linux-x86_64-2.7/xen/lowlevel/xc/xc.o -fno-strict-aliasing 
> >> -Werror
> >> In file included from /usr/include/limits.h:25:0,
> >>                  from 
> >> /usr/lib/gcc/x86_64-redhat-linux/4.9.2/include/limits.h:168,
> >>                  from 
> >> /usr/lib/gcc/x86_64-redhat-linux/4.9.2/include/syslimits.h:7,
> >>                  from 
> >> /usr/lib/gcc/x86_64-redhat-linux/4.9.2/include/limits.h:34,
> >>                  from /usr/include/python2.7/Python.h:19,
> >>                  from xen/lowlevel/xc/xc.c:7:
> >> /usr/include/features.h:328:4: error: #warning _FORTIFY_SOURCE requires 
> >> compiling with optimization (-O) [-Werror=cpp]
> > 
> > Where is _FORTIFY_SOURCE coming from? I don't see it in our tree
> > anywhere except stubdom/Makefile which is disabling it and the build
> > worked for me. Perhaps it is coming from your build environment
> > somewhere? How are you configuring and building Xen?
> 
> _FORTIFY_SOURCE is just for python, and it comes from 
> /usr/lib64/python2.7/config/Makefile:
> 
> # Compiler options
> OPT=            -DNDEBUG -O2 -g -pipe -Wall -Werror=format-security 
> -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong 
> --param=ssp-buffer-size=4 -grecord-gcc-switches  -m64 -mtune=generic 
> -D_GNU_SOURCE -fPIC -fwrapv
> BASECFLAGS=      -fno-strict-aliasing
> CFLAGS=         $(BASECFLAGS) -O2 -g -pipe -Wall -Werror=format-security 
> -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector-strong 
> --param=ssp-buffer-size=4 -grecord-gcc-switches  -m64 -mtune=generic 
> -D_GNU_SOURCE -fPIC -fwrapv  $(OPT) $(EXTRA_CFLAGS)
> 

That's a bit antisocial of it...

I can't reproduce this on Debian with Python 2.7. _FORTIFY_SOURCE=2 is
not passed when building the Python bits (it is passed when linking,
oddly). Debian's /usr/lib/python2.7/config-x86_64-linux-gnu/Makefile
looks rather different to what you quote above though, it avoids bare
CFLAGS for one thing (reserving them for the user).

I have a suspicion that this might be a bug in the Fedora Python
packaging, in that they have leaked inappropriate build-time flags from
the actual Python build into the set of flags to be used to build 3rd
party Python modules.

Please can you check this with the Fedora folks? If they respond that
this is deliberate etc then please let us know and we'll see what we can
do in Xen to work around it.

Ian.


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