Re: [PATCH] lkdtm: support llvm-objcopy

[Nathan Chancellor <natechancellor@gmail.com>]

On Mon, May 13, 2019 at 04:38:54PM -0700, Jordan Rupprecht wrote:
> Nathan: is your version of llvm-objcopy later than r359639 (April 30)?
> 
> 
> From: Nathan Chancellor <natechancellor@gmail.com>
> Date: Mon, May 13, 2019 at 4:29 PM
> To: Nick Desaulniers
> Cc: <keescook@chromium.org>, <clang-built-linux@googlegroups.com>,
> Nathan Chancellor, Jordan Rupprect, Arnd Bergmann, Greg Kroah-Hartman,
> <linux-kernel@vger.kernel.org>
> 
> > On Mon, May 13, 2019 at 03:21:09PM -0700, 'Nick Desaulniers' via Clang Built Linux wrote:
> > > With CONFIG_LKDTM=y and make OBJCOPY=llvm-objcopy, llvm-objcopy errors:
> > > llvm-objcopy: error: --set-section-flags=.text conflicts with
> > > --rename-section=.text=.rodata
> > >
> > > Rather than support setting flags then renaming sections vs renaming
> > > then setting flags, it's simpler to just change both at the same time
> > > via --rename-section.
> > >
> > > This can be verified with:
> > > $ readelf -S drivers/misc/lkdtm/rodata_objcopy.o
> > > ...
> > > Section Headers:
> > >   [Nr] Name              Type             Address           Offset
> > >        Size              EntSize          Flags  Link  Info  Align
> > > ...
> > >   [ 1] .rodata           PROGBITS         0000000000000000  00000040
> > >        0000000000000004  0000000000000000   A       0     0     4
> > > ...
> > >
> > > Which shows in the Flags field that .text is now renamed .rodata, the
> > > append flag A is set, and the section is not flagged as writeable W.
> > >
> > > Link: https://github.com/ClangBuiltLinux/linux/issues/448
> > > Reported-by: Nathan Chancellor <nathanchance@gmail.com>
> >
> > This should be natechancellor@gmail.com (although I think I do own that
> > email, just haven't been into it for 10+ years...)
> >
> > > Suggested-by: Jordan Rupprect <rupprecht@google.com>
> > > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> > > ---
> > >  drivers/misc/lkdtm/Makefile | 3 +--
> > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/misc/lkdtm/Makefile b/drivers/misc/lkdtm/Makefile
> > > index 951c984de61a..89dee2a9d88c 100644
> > > --- a/drivers/misc/lkdtm/Makefile
> > > +++ b/drivers/misc/lkdtm/Makefile
> > > @@ -15,8 +15,7 @@ KCOV_INSTRUMENT_rodata.o    := n
> > >
> > >  OBJCOPYFLAGS :=
> > >  OBJCOPYFLAGS_rodata_objcopy.o        := \
> > > -                     --set-section-flags .text=alloc,readonly \
> > > -                     --rename-section .text=.rodata
> > > +                     --rename-section .text=.rodata,alloc,readonly
> > >  targets += rodata.o rodata_objcopy.o
> > >  $(obj)/rodata_objcopy.o: $(obj)/rodata.o FORCE
> > >       $(call if_changed,objcopy)
> > > --
> > > 2.21.0.1020.gf2820cf01a-goog
> > >
> >
> > I ran this script to see if there was any change for GNU objcopy and it
> > looks like .rodata's type gets changed, is this intentional? Otherwise,
> > this works for llvm-objcopy like you show.
> >
> > -----------
> >
> > 1c1
> > < There are 11 section headers, starting at offset 0x240:
> > ---
> > > There are 11 section headers, starting at offset 0x230:
> > 8c8
> > <   [ 1] .rodata           PROGBITS         0000000000000000  00000040
> > ---
> > >   [ 1] .rodata           NOBITS           0000000000000000  00000040
> > 10c10
> >
> > -----------
> >
> > #!/bin/bash
> >
> > TMP1=$(mktemp)
> > TMP2=$(mktemp)
> >
> > git checkout next-20190513
> >
> > make -j$(nproc) ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- O=out mrproper allyesconfig drivers/misc/lkdtm/
> > readelf -S out/drivers/misc/lkdtm/rodata_objcopy.o > ${TMP1}
> >
> > curl -LSs https://lore.kernel.org/lkml/20190513222109.110020-1-ndesaulniers@google.com/raw | git am -3
> >
> > make -j$(nproc) ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- O=out mrproper allyesconfig drivers/misc/lkdtm/
> > readelf -S out/drivers/misc/lkdtm/rodata_objcopy.o > ${TMP2}
> >
> > diff ${TMP1} ${TMP2}

Hi Jordan,

Yes but that output was purely with GNU objcopy (checking section
headers before and after this patch). This is the section header
for llvm-objcopy after this patch:

  [ 1] .rodata           PROGBITS         0000000000000000  00000040
         0000000000000004  0000000000000000   A       0     0     4

Cheers,
Nathan