Re: [PATCH 2/2] mingw: enable DEP and ASLR

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

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Hi Alban,

On Wed, 1 May 2019, Alban Gruin wrote:

> Le 01/05/2019 à 00:41, Johannes Schindelin a écrit :
> >
> > On Tue, 30 Apr 2019, Johannes Sixt wrote:
> >
> >> [had to add Dscho as recipient manually, mind you]
> >
> > I usually pick up responses to GitGitGadget patch series even if I am not
> > on explicit Cc: (but it might take a couple of days when I am too busy
> > elsewhere to read the Git mailing list).
> >
> >> Am 29.04.19 um 23:56 schrieb İsmail Dönmez via GitGitGadget:
> >>> From: =?UTF-8?q?=C4=B0smail=20D=C3=B6nmez?= <ismail@i10z.com>
> >>>
> >>> Enable DEP (Data Execution Prevention) and ASLR (Address Space Layout
> >>> Randomization) support. This applies to both 32bit and 64bit builds
> >>> and makes it substantially harder to exploit security holes in Git by
> >>> offering a much more unpredictable attack surface.
> >>>
> >>> ASLR interferes with GDB's ability to set breakpoints. A similar issue
> >>> holds true when compiling with -O2 (in which case single-stepping is
> >>> messed up because GDB cannot map the code back to the original source
> >>> code properly). Therefore we simply enable ASLR only when an
>
> I don’t know if it stands true when combined with something like -ggdb3,
> but I may be very wrong.  Feel free to correct me.

Possibly, but that makes my job here harder, so I won't even try right now
;-)

> >>> optimization flag is present in the CFLAGS, using it as an indicator
> >>> that the developer does not want to debug in GDB anyway.
> >>>
> >>> Signed-off-by: İsmail Dönmez <ismail@i10z.com>
> >>> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> >>> ---
> >>>  config.mak.uname | 6 ++++++
> >>>  1 file changed, 6 insertions(+)
> >>>
> >>> diff --git a/config.mak.uname b/config.mak.uname
> >>> index e7c7d14e5f..a9edcc5f0b 100644
> >>> --- a/config.mak.uname
> >>> +++ b/config.mak.uname
> >>> @@ -570,6 +570,12 @@ else
> >>>  	ifeq ($(shell expr "$(uname_R)" : '2\.'),2)
> >>>  		# MSys2
> >>>  		prefix = /usr/
> >>> +		# Enable DEP
> >>> +		BASIC_LDFLAGS += -Wl,--nxcompat
> >>> +		# Enable ASLR (unless debugging)
> >>> +		ifneq (,$(findstring -O,$(CFLAGS)))
> >>> +			BASIC_LDFLAGS += -Wl,--dynamicbase
> >>> +		endif
> >>>  		ifeq (MINGW32,$(MSYSTEM))
> >>>  			prefix = /mingw32
> >>>  			HOST_CPU = i686
> >>>
> >>
> >> I'm a bit concerned that this breaks my debug sessions where I use -O0.
> >> But I'll test without -O0 before I really complain.
> >
> > Weird. Jameson Miller also mentioned this very concern in an internal
> > review.
> >
> > I guess I'll do something like
> >
> > 	ifneq (,$(findstring -O,$(filter-out -O0,$(CFLAGS))))
> >
>
> -Og also exists to debug[0], even if it’s far less known.

Good point.

> Perhaps it’s better to check for -g (and its variants[1]) as the user
> clearly states their intent to debug the resulting binary, rather than
> checking for special cases.

I don't think we can use that, as we specifically build Git for Windows
with optimization *and* with debug symbols (and then use cv2pdb to extract
those debug symbols into external .pdb files for use with advanced
post-mortem tools, i.e. we do *not* need to single-step).

Thanks,
Dscho