Re: [PATCH 2/2] x86: Discard .note.gnu.property sections in vmlinux

[H.J. Lu]

On Thu, Jan 30, 2020 at 12:08 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Thu, Jan 30, 2020 at 12:04:54PM -0800, H.J. Lu wrote:
> > > I don't understand this. "may not be incompatible"? Is there an error
> > > generated? If so, what does it look like?
> >
> > When -mx86-used-note=yes is passed to assembler, with my patch, I got
> >
> > [hjl@gnu-skx-1 linux]$ readelf -n vmlinux
> >
> > Displaying notes found in: .notes
> >   Owner                Data size Description
> >   Xen                  0x00000006 Unknown note type: (0x00000006)
> >    description data: 6c 69 6e 75 78 00
> >   Xen                  0x00000004 Unknown note type: (0x00000007)
> >    description data: 32 2e 36 00
> >   Xen                  0x00000008 Unknown note type: (0x00000005)
> >    description data: 78 65 6e 2d 33 2e 30 00
> >   Xen                  0x00000008 Unknown note type: (0x00000003)
> >    description data: 00 00 00 80 ff ff ff ff
> >   Xen                  0x00000008 Unknown note type: (0x0000000f)
> >    description data: 00 00 00 00 80 00 00 00
> >   Xen                  0x00000008 NT_VERSION (version)
> >    description data: 80 a1 ba 82 ff ff ff ff
> >   Xen                  0x00000008 NT_ARCH (architecture)
> >    description data: 00 10 00 81 ff ff ff ff
> >   Xen                  0x00000029 Unknown note type: (0x0000000a)
> >    description data: 21 77 72 69 74 61 62 6c 65 5f 70 61 67 65 5f 74
> > 61 62 6c 65 73 7c 70 61 65 5f 70 67 64 69 72 5f 61 62 6f 76 65 5f 34
> > 67 62
> >   Xen                  0x00000004 Unknown note type: (0x00000011)
> >    description data: 01 88 00 00
> >   Xen                  0x00000004 Unknown note type: (0x00000009)
> >    description data: 79 65 73 00
> >   Xen                  0x00000008 Unknown note type: (0x00000008)
> >    description data: 67 65 6e 65 72 69 63 00
> >   Xen                  0x00000010 Unknown note type: (0x0000000d)
> >    description data: 01 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00
> >   Xen                  0x00000004 Unknown note type: (0x0000000e)
> >    description data: 01 00 00 00
> >   Xen                  0x00000004 Unknown note type: (0x00000010)
> >    description data: 01 00 00 00
> >   Xen                  0x00000008 Unknown note type: (0x0000000c)
> >    description data: 00 00 00 00 00 80 ff ff
> >   Xen                  0x00000008 Unknown note type: (0x00000004)
> >    description data: 00 00 00 00 00 00 00 00
> >   GNU                  0x00000014 NT_GNU_BUILD_ID (unique build ID bitstring)
> >     Build ID: 11c73de2922f593e1b35b92ab3c70eaa1a80fa83
> >   Linux                0x00000018 OPEN
> >    description data: 35 2e 33 2e 39 2d 32 30 30 2e 30 2e 66 63 33 30
> > 2e 78 38 36 5f 36 34 00
> >   Xen                  0x00000008 Unknown note type: (0x00000012)
> >    description data: 70 04 00 01 00 00 00 00
> > [hjl@gnu-skx-1 linux]$
> >
> > Without my patch,
> >
> > [hjl@gnu-skx-1 linux]$ readelf -n vmlinux
> >
> > Displaying notes found in: .notes
> >   Owner                Data size Description
> >   Xen                  0x00000006 Unknown note type: (0x00000006)
> >    description data: 6c 69 6e 75 78 00
> >   Xen                  0x00000004 Unknown note type: (0x00000007)
> >    description data: 32 2e 36 00
> >   xen-3.0              0x00000005 Unknown note type: (0x006e6558)
> >    description data: 08 00 00 00 03
> > readelf: Warning: note with invalid namesz and/or descsz found at offset 0x50
> > readelf: Warning:  type: 0xffffffff, namesize: 0x006e6558, descsize:
> > 0x80000000, alignment: 8
> > [hjl@gnu-skx-1 linux]$
>
> What is the source of this failure? Does readelf need updating instead?
> Is the linking step producing an invalid section? It seems like
> discarding the properties isn't the right solution here?

With the command-line option, -mx86-used-note=yes, the x86 assembler
in binutils 2.32 and above generates a program property note in a note
section, .note.gnu.property, to encode used x86 ISAs and features.
But x86 kernel linker script only contains a signle NOTE segment:

PHDRS {
 text PT_LOAD FLAGS(5);
 data PT_LOAD FLAGS(6);
 percpu PT_LOAD FLAGS(6);
 init PT_LOAD FLAGS(7);
 note PT_NOTE FLAGS(0);
}
SECTIONS
{
...
 .notes : AT(ADDR(.notes) - 0xffffffff80000000) { __start_notes = .; KEEP(*(.not
e.*)) __stop_notes = .; } :text :note
...
}

But .note.gnu.property must be 8-byte aligned.  Linker deals with it
by generating
2 PT_NOTE segments, one has 4-byte alignment and the other has 8-byte alignment:

[hjl@gnu-cfl-1 ~]$ readelf -l /usr/local/bin/ld

Elf file type is EXEC (Executable file)
Entry point 0x404530
There are 13 program headers, starting at offset 64

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000040 0x0000000000400040 0x0000000000400040
                 0x00000000000002d8 0x00000000000002d8  R      0x8
  INTERP         0x0000000000000318 0x0000000000400318 0x0000000000400318
                 0x000000000000001c 0x000000000000001c  R      0x1
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                 0x0000000000002a30 0x0000000000002a30  R      0x1000
  LOAD           0x0000000000003000 0x0000000000403000 0x0000000000403000
                 0x00000000000d7b35 0x00000000000d7b35  R E    0x1000
  LOAD           0x00000000000db000 0x00000000004db000 0x00000000004db000
                 0x0000000000179248 0x0000000000179248  R      0x1000
  LOAD           0x0000000000254de0 0x0000000000655de0 0x0000000000655de0
                 0x00000000000062e8 0x000000000000ba68  RW     0x1000
  DYNAMIC        0x0000000000254df0 0x0000000000655df0 0x0000000000655df0
                 0x0000000000000200 0x0000000000000200  RW     0x8
  NOTE           0x0000000000000338 0x0000000000400338 0x0000000000400338
                 0x0000000000000030 0x0000000000000030  R      0x8
  NOTE           0x0000000000000368 0x0000000000400368 0x0000000000400368
                 0x0000000000000044 0x0000000000000044  R      0x4
  GNU_PROPERTY   0x0000000000000338 0x0000000000400338 0x0000000000400338
                 0x0000000000000030 0x0000000000000030  R      0x8
  GNU_EH_FRAME   0x0000000000233efc 0x0000000000633efc 0x0000000000633efc
                 0x000000000000478c 0x000000000000478c  R      0x4
  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x0000000000000000 0x0000000000000000  RW     0x10
  GNU_RELRO      0x0000000000254de0 0x0000000000655de0 0x0000000000655de0
                 0x0000000000000220 0x0000000000000220  R      0x1

 Section to Segment mapping:
  Segment Sections...
   00
   01     .interp
   02     .interp .note.gnu.property .note.gnu.build-id .note.ABI-tag
.hash .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn
.rela.plt
   03     .init .plt .text .fini
   04     .rodata .eh_frame_hdr .eh_frame
   05     .init_array .fini_array .dynamic .got .got.plt .data .bss
   06     .dynamic
   07     .note.gnu.property
   08     .note.gnu.build-id .note.ABI-tag
   09     .note.gnu.property
   10     .eh_frame_hdr
   11
   12     .init_array .fini_array .dynamic .got
[hjl@gnu-cfl-1 ~]$

Since .note.gnu.property in vmlinux is unused, it can be discarded.

-- 
H.J.