Re: [python] [PATCH v3 1/5] x86/boot: create a C bundle for 32 bit boot code and use it

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 11/10/2024 9:52 am, Frediano Ziglio wrote:
> diff --git a/xen/arch/x86/boot/Makefile b/xen/arch/x86/boot/Makefile
> index ff0f965876..4cf0d7e140 100644
> --- a/xen/arch/x86/boot/Makefile
> +++ b/xen/arch/x86/boot/Makefile
> ...
> +$(obj)/built_in_32.o: $(obj)/built_in_32.other.bin 
> $(obj)/built_in_32.final.bin
> +     $(PYTHON) $(srctree)/tools/combine_two_binaries \
> +             --script $(obj)/build32.final.lds \
> +             --bin1 $(obj)/built_in_32.other.bin --bin2 
> $(obj)/built_in_32.final.bin \
> +             --map $(obj)/built_in_32.final.map \
> +             --exports cmdline_parse_early,reloc \
> +             --section-header '.section .init.text, "ax", @progbits' \
> +             --output $(obj)/built_in_32.s

I can't see a case where we'd want this anywhere other than .init.text,
so I'd drop the --section-header and just write it out unconditionally. 
However, looking at the output:

xen$ head arch/x86/boot/built_in_32.S
    .section .init.text, "ax", @progbits
    .byte 137,194,128,56,0,116,6,64,128,56,0,117,250,41,208,195
    .byte 133,201,116,42,86,83,141,52,8,64,15,182,72,255,66,15
    .byte 182,90,255,56,217,117,15,132,201,116,25,57,198,117,234,184

This wants to start with a comment saying that it was automatically
generated by combine_two_binaries.

Next, we need some kind of symbol at the start.  Right now, the
disassembly reads:

    arch/x86/boot/built_in_32.o:     file format elf64-x86-64
    Disassembly of section .init.text:
    0000000000000000 <cmdline_parse_early-0x391>:
       0:   89 c2                   mov    %eax,%edx
       2:   80 38 00                cmpb   $0x0,(%eax)


because most metadata is lost by this transformation, and it doesn't
know that this is in fact strlen().  I'd suggest suggest obj32_start: or
equivalent.

> diff --git a/xen/arch/x86/boot/build32.lds b/xen/arch/x86/boot/build32.lds.S
> similarity index 63%
> rename from xen/arch/x86/boot/build32.lds
> rename to xen/arch/x86/boot/build32.lds.S
> index 56edaa727b..72a4c5c614 100644
> --- a/xen/arch/x86/boot/build32.lds
> +++ b/xen/arch/x86/boot/build32.lds.S
> @@ -15,22 +15,52 @@
>   * with this program.  If not, see <http://www.gnu.org/licenses/>.
>   */
>  
> -ENTRY(_start)
> +#ifdef FINAL
> +# define GAP 0
> +# define MULT 0
> +# define TEXT_START
> +#else
> +# define GAP 0x010200
> +# define MULT 1
> +# define TEXT_START 0x408020
> +#endif
> +# define DECLARE_IMPORT(name) name = . + (__LINE__ * MULT)
> +
> +ENTRY(dummy_start)
>  
>  SECTIONS
>  {
> -  /* Merge code and data into one section. */
> -  .text : {
> +  /* Merge code and read-only data into one section. */
> +  .text TEXT_START : {
> +        /* Silence linker warning, we are not going to use it */
> +        dummy_start = .;
> +
> +        /* Declare below any symbol name needed.
> +         * Each symbol should be on its own line.
> +         * It looks like a tedious work but we make sure the things we use.
> +         * Potentially they should be all variables. */
> +        DECLARE_IMPORT(__base_relocs_start);
> +        DECLARE_IMPORT(__base_relocs_end);
> +        . = . + GAP;
>          *(.text)
>          *(.text.*)
> -        *(.data)
> -        *(.data.*)
>          *(.rodata)
>          *(.rodata.*)
> +  }
> +
> +  /* Writeable data sections. Check empty.
> +   * We collapse all into code section and we don't want it to be writeable. 
> */
> +  .data : {
> +        *(.data)
> +        *(.data.*)
>          *(.bss)
>          *(.bss.*)
>    }
> -
> +  /DISCARD/ : {
> +       *(.comment)
> +       *(.comment.*)
> +       *(.note.*)
> +  }
>    /* Dynamic linkage sections.  Collected simply so we can check they're 
> empty. */
>    .got : {
>          *(.got)
> @@ -64,3 +94,4 @@ ASSERT(SIZEOF(.igot.plt) == 0,    ".igot.plt non-empty")
>  ASSERT(SIZEOF(.iplt) == 0,        ".iplt non-empty")
>  ASSERT(SIZEOF(.plt) == 0,         ".plt non-empty")
>  ASSERT(SIZEOF(.rel) == 0,         "leftover relocations")
> +ASSERT(SIZEOF(.data) == 0,        "we don't want data")

3mdeb are getting around to rebasing/resubmitting the Trenchboot work
(Intel TXT and AMD SKINIT) backing QubeOS Anti-Evil-Maid.

While most of the cleanup is proving very helpful (i.e. reducing their
work), the lack of .data was seen as likely to be a blocker.  Thinking
about this more, I'm now fairly certain we do not need to exclude data.

This object executes during boot in 32bit flat unpaged mode (i.e. there
are no actual restrictions during execution), and because it's all
wrapped in .init.text, its "just code" to the outside world.  This means
it does not impact R^X that we're trying to arrange for the EFI section
headers.

Therefore the data arrangements should stay as they were before, and I
think the result will be fine.  We obviously don't want gratuitous use
of .data, but we don't need to prohibit it either.

I've got various other suggestions for improvements of the result, but
they can all be deferred until later.  This is complicated enough.

> diff --git a/xen/tools/combine_two_binaries b/xen/tools/combine_two_binaries
> new file mode 100755
> index 0000000000..ea2d6ddc4e
> --- /dev/null
> +++ b/xen/tools/combine_two_binaries
> @@ -0,0 +1,198 @@
> +#!/usr/bin/env python3
> +
> +from __future__ import print_function
> +import argparse
> +import re
> +import struct
> +import sys
> +
> +parser = argparse.ArgumentParser(description='Generate assembly file to 
> merge into other code.')
> +parser.add_argument('--script', dest='script',
> +                    required=True,
> +                    help='Linker script for extracting symbols')
> +parser.add_argument('--bin1', dest='bin1',
> +                    required=True,
> +                    help='First binary')
> +parser.add_argument('--bin2', dest='bin2',
> +                    required=True,
> +                    help='Second binary')
> +parser.add_argument('--output', dest='output',
> +                    help='Output file')
> +parser.add_argument('--map', dest='mapfile',
> +                    help='Map file to read for symbols to export')
> +parser.add_argument('--exports', dest='exports',
> +                    help='Symbols to export')
> +parser.add_argument('--section-header', dest='section_header',
> +                    default='.text',
> +                    help='Section header declaration')
> +parser.add_argument('-v', '--verbose',
> +                    action='store_true')
> +args = parser.parse_args()
> +
> +gap = 0x010200
> +text_diff = 0x408020
> +
> +# Parse linker script for external symbols to use.
> +symbol_re = re.compile(r'\s+(\S+)\s*=\s*\.\s*\+\s*\((\d+)\s*\*\s*0\s*\)\s*;')

What is this looking for?  I'd expect the DECLARE_IMPORT() lines, but
this is as clear as regexes...

> +symbols = {}
> +lines = {}
> +for line in open(args.script):
> +    m = symbol_re.match(line)
> +    if not m:
> +        continue
> +    (name, line_num) = (m.group(1), int(m.group(2)))
> +    if line_num == 0:
> +        raise Exception("Invalid line number found:\n\t" + line)
> +    if line_num in symbols:
> +        raise Exception("Symbol with this line already present:\n\t" + line)
> +    if name in lines:
> +        raise Exception("Symbol with this name already present:\n\t" + name)
> +    symbols[line_num] = name
> +    lines[name] = line_num
> +
> +exports = []
> +if args.exports is not None:
> +    exports = dict([(name, None) for name in args.exports.split(',')])
> +
> +# Parse mapfile, look for ther symbols we want to export.
> +if args.mapfile is not None:
> +    symbol_re = re.compile(r'\s{15,}0x([0-9a-f]+)\s+(\S+)\n')
> +    for line in open(args.mapfile):
> +        m = symbol_re.match(line)
> +        if not m or m.group(2) not in exports:
> +            continue
> +        addr = int(m.group(1), 16)
> +        exports[m.group(2)] = addr
> +for (name, addr) in exports.items():
> +    if addr is None:
> +        raise Exception("Required export symbols %s not found" % name)
> +
> +file1 = open(args.bin1, 'rb')
> +file2 = open(args.bin2, 'rb')
> +file1.seek(0, 2)
> +size1 = file1.tell()
> +file2.seek(0, 2)
> +size2 = file2.tell()
> +if size1 > size2:
> +    file1, file2 = file2, file1
> +    size1, size2 = size2, size1
> +if size2 != size1 + gap:
> +    raise Exception('File sizes do not match')
> +
> +file1.seek(0, 0)
> +data1 = file1.read(size1)
> +file2.seek(gap, 0)
> +data2 = file2.read(size1)
> +
> +max_line = max(symbols.keys())
> +
> +def to_int32(n):
> +    '''Convert a number to signed 32 bit integer truncating if needed'''
> +    mask = (1 << 32) - 1
> +    h = 1 << 31
> +    return (n & mask) ^ h - h
> +
> +i = 0
> +references = {}
> +internals = 0
> +while i <= size1 - 4:
> +    n1 = struct.unpack('<I', data1[i:i+4])[0]
> +    n2 = struct.unpack('<I', data2[i:i+4])[0]
> +    i += 1
> +    # The two numbers are the same, no problems
> +    if n1 == n2:
> +        continue
> +    # Try to understand why they are different
> +    diff = to_int32(n1 - n2)
> +    if diff == -gap: # this is an internal relocation
> +        pos = i - 1
> +        print(("Internal relocation found at position %#x "
> +               "n1=%#x n2=%#x diff=%#x") % (pos, n1, n2, diff),

Here and elsewhere, you don't need brackets around around the string
itself.  Python strings are like C strings and will auto-concatenate.

> +              file=sys.stderr)
> +        i += 3
> +        internals += 1
> +        if internals >= 10:
> +            break
> +        continue
> +    # This is a relative relocation to a symbol, accepted, code/data is
> +    # relocatable.
> +    if diff < gap and diff >= gap - max_line:
> +        n = gap - diff
> +        symbol = symbols.get(n)
> +        # check we have a symbol
> +        if symbol is None:
> +            raise Exception("Cannot find symbol for line %d" % n)
> +        pos = i - 1
> +        if args.verbose:
> +            print('Position %#x %d %s' % (pos, n, symbol), file=sys.stderr)
> +        i += 3
> +        references[pos] = symbol
> +        continue
> +    # First byte is the same, move to next byte
> +    if diff & 0xff == 0 and i <= size1 - 4:
> +       continue
> +    # Probably a type of relocation we don't want or support
> +    pos = i - 1
> +    suggestion = ''
> +    symbol = symbols.get(-diff - text_diff)
> +    if symbol is not None:
> +        suggestion = " Maybe %s is not defined as hidden?" % symbol
> +    raise Exception(("Unexpected difference found at %#x "
> +                     "n1=%#x n2=%#x diff=%#x gap=%#x.%s") % \
> +                     (pos, n1, n2, diff, gap, suggestion))
> +if internals != 0:
> +    raise Exception("Previous relocations found")
> +
> +def line_bytes(buf, out):
> +    '''Output an assembly line with all bytes in "buf"'''
> +    if type(buf) == str:
> +        print("\t.byte " + ','.join([str(ord(c)) for c in buf]), file=out)
> +    else:
> +        print("\t.byte " + ','.join([str(n) for n in buf]), file=out)

I'm guessing this is Py2/3 compatibility?

> +
> +def part(start, end, out):
> +    '''Output bytes of "data" from "start" to "end"'''
> +    while start < end:
> +        e = min(start + 16, end)
> +        line_bytes(data1[start:e], out)
> +        start = e
> +
> +def reference(pos, out):
> +    name = references[pos]
> +    n = struct.unpack('<I', data1[pos:pos+4])[0]
> +    sign = '+'
> +    if n >= (1 << 31):
> +        n -= (1 << 32)
> +    n += pos
> +    if n < 0:
> +        n = -n
> +        sign = '-'
> +    print("\t.hidden %s\n\t.long %s %s %#x - ." % (name, name, sign, n),

Personally, I think this is easier to read as:

    print("\t.hidden %s\n"
          "\t.long %s %s %#x - ." % (name, name, sign, n),
          file=out)

so it visually matches the output too.  Same for .globl/hidden lower.

~Andrew