Re: [PATCH] mini-os: Avoid segfaults in tc{g,s}etattr

[Wei Liu <wl@xen.org>]

On Mon, Apr 27, 2020 at 09:30:50AM -0400, Jason Andryuk wrote:
> On Mon, Apr 27, 2020 at 3:54 AM Samuel Thibault
> <samuel.thibault@ens-lyon.org> wrote:
> >
> > Jason Andryuk, le dim. 26 avril 2020 23:40:19 -0400, a ecrit:
> > > Commit c96c22f1d94 "mini-os: minimal implementations of some termios
> > > functions" introduced implementations of tcgetattr and tcsetattr.
> > > However, they do not check if files[fildes].cons.dev is non-NULL before
> > > dereferencing.  This is not a problem for FDs allocated through
> > > alloc_fd, but the files array pre-allocates FDs 0-2 for stdio.  Those
> > > entries have a NULL .dev, so tc{g,s}etattr on them segfault.
> > >
> > > ioemu-stubdom segfaults when term_init() calls tcgetattr on FD 0.
> > >
> > > Restore tcgetattr and tcsetattr behavior when .dev is NULL equivalent to
> > > unsupported_function as it was before c96c22f1d94.
> > >
> > > Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
> >
> > Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
> >
> > Thanks!
> 
> Thank you!
> 
> > > ---
> > > I can't get ioemu-stubdom to start without this.  With this, the guest
> > > just reboots immediately, but it does that with a non-stubdom
> > > device_model_version="qemu-xen-traditional" .  The same guest disk image
> > > (cirros 0.5.1) boots with a linux stubdom or non-stubdom Ubuntu
> > > qemu-system-x86_64.
> 
> Ubuntu gcc-9 adds -fcf-protection by default.  Somehow that flag
> caused rombios (I think) to restart.  Setting -fcf-protection=none
> like below (probably just the EMBEDDED_EXTRA_CFLAGS part) lets rombios
> start properly.  The hypervisor needs it as well via
> EXTRA_CFLAGS_XEN_CORE=-fcf-protection=none and maybe also added to
> xen/arch/x86/boot/build32.mk .

Are you able to turn this into a proper patch? I suspect you will need
to test the availability of this new (?) flag.

Also Cc Jan and Andrew because it affects hypervisor build too.

> 
> diff --git a/Config.mk b/Config.mk
> index 0f303c79b2..efb3d42bc4 100644
> --- a/Config.mk
> +++ b/Config.mk
> @@ -205,6 +205,7 @@ APPEND_CFLAGS += $(foreach i, $(APPEND_INCLUDES), -I$(i))
> 
>  EMBEDDED_EXTRA_CFLAGS := -nopie -fno-stack-protector -fno-stack-protector-all
>  EMBEDDED_EXTRA_CFLAGS += -fno-exceptions
> +EMBEDDED_EXTRA_CFLAGS += -fcf-protection=none
> 
>  XEN_EXTFILES_URL ?= http://xenbits.xen.org/xen-extfiles
>  # All the files at that location were downloaded from elsewhere on
> diff --git a/tools/firmware/Rules.mk b/tools/firmware/Rules.mk
> index 26bbddccd4..0d33514d53 100644
> --- a/tools/firmware/Rules.mk
> +++ b/tools/firmware/Rules.mk
> @@ -17,3 +17,4 @@ $(call cc-options-add,CFLAGS,CC,$(EMBEDDED_EXTRA_CFLAGS))
> 
>  # Extra CFLAGS suitable for an embedded type of environment.
>  CFLAGS += -fno-builtin -msoft-float
> +CFLAGS += -fcf-protection=none
>