Re: [PATCH v3 3/3] vdso: Reject absolute relocations during build

[Thomas Gleixner <tglx@linutronix.de>]

On Thu, Jun 12 2025 at 16:21, Thomas Weißschuh wrote:
> On Thu, Jun 12, 2025 at 10:31:20AM +0200, Alexandre Ghiti wrote:
> We could introduce per-architecture configuration. Essentially reverting parts
> of commit aff69273af61 ("vdso: Improve cmd_vdso_check to check all dynamic relocations").
> The final logic for the intermediary objects still needs to be more complicated
> than for the final .so as those contain relocations in the debug information.
>
> Or we could add a C hostprog for validation.
> That would be much more flexible than the inline shell command.
> It would then also be easier to use an allow-list than the brittle deny-list.
>
> Or we don't do anything, relying on the selftests to detect miscompilations.

That's a bad idea :)

> I'll run this by tglx. If somebody else has any opinions, I'm all ears.

This is all a mess because the relocation type numbers and their R_*
names are not uniform accross architectures. Neither are the valid
relocation types which are suitable for VDSO.

I don't think you can reasonably cover all of it with readelf and
grep. I did some unrelated relocation analysis some time ago and I just
modified the python script (yes, I hate to use libelf) to show case how
insane this gets. This is just as much as I needed to analyse files
compiled with some random cross gcc I had handy. But you surely get the
idea.

Thanks,

        tglx
---
#!/usr/bin/env python3

import sys

from argparse import ArgumentParser
from elftools.elf.elffile import ELFFile
from elftools.elf.relocation import RelocationSection
from elftools.elf.enums import ENUM_RELOC_TYPE_i386, ENUM_RELOC_TYPE_x64
from elftools.elf.enums import ENUM_RELOC_TYPE_ARM, ENUM_RELOC_TYPE_AARCH64
from elftools.elf.descriptions import describe_reloc_type

class relocs(object):
    def __init__(self, arch, sections, types):
        self.arch = arch
        self.sections = sections
        self.types = types

i386_relocs = relocs('EM_386',
                     [ '.rel.text' ],
                     [ ENUM_RELOC_TYPE_i386['R_386_NONE'],
                       ENUM_RELOC_TYPE_i386['R_386_PC32'],
                       ENUM_RELOC_TYPE_i386['R_386_GOTPC'],
                       ENUM_RELOC_TYPE_i386['R_386_GOTOFF'],
                      ])

x86_64_relocs = relocs('EM_X86_64',
                       [ '.rela.text' ],
                       [ ENUM_RELOC_TYPE_x64['R_X86_64_NONE'],
                         ENUM_RELOC_TYPE_x64['R_X86_64_PC32'],
                        ])

arm_relocs = relocs('EM_ARM',
                       [ '.rela.text' ],
                       # Probably incomplete
                       [ ENUM_RELOC_TYPE_ARM['R_ARM_NONE'],
                         ENUM_RELOC_TYPE_ARM['R_ARM_REL32'],
                        ])

arm64_relocs = relocs('EM_AARCH64',
                       [ '.rela.text' ],
                       # Probably incomplete
                       [ ENUM_RELOC_TYPE_AARCH64['R_AARCH64_NONE'],
                         ENUM_RELOC_TYPE_AARCH64['R_AARCH64_ADR_PREL_LO21'],
                        ])

# Minimal set for an example VDSO build
ENUM_RELOC_TYPE_RISCV = dict(
    R_RISCV_BRANCH        = 0x10,
    R_RISCV_PCREL_HI20    = 0x17,
    R_RISCV_PCREL_LO12_I  = 0x18,
    R_RISCV_RVC_BRANCH    = 0x2c,
    R_RISCV_RVC_JUMP      = 0x2d,
    R_RISCV_RELAX         = 0x33,
)

riscv_relocs = relocs('EM_RISCV',
                       [ '.rela.text' ],
                       [ ENUM_RELOC_TYPE_RISCV['R_RISCV_BRANCH'],
                         ENUM_RELOC_TYPE_RISCV['R_RISCV_PCREL_HI20'],
                         ENUM_RELOC_TYPE_RISCV['R_RISCV_PCREL_LO12_I'],
                         ENUM_RELOC_TYPE_RISCV['R_RISCV_RVC_BRANCH'],
                         ENUM_RELOC_TYPE_RISCV['R_RISCV_RVC_JUMP'],
                         ENUM_RELOC_TYPE_RISCV['R_RISCV_RELAX'],
                        ])
supported_archs = {
    'i386'      : i386_relocs,
    'x86_64'    : x86_64_relocs,
    'arm'       : arm_relocs,
    'arm64'     : arm64_relocs,
    'riscv'     : riscv_relocs,
}

# Probably incomplete
invalid_relocs = [ '.rela.dyn', '.rela.plt' ]

def check_relocations(file, arch):
    elf = ELFFile(file)
    res = 0

    if elf.header['e_machine'] != arch.arch:
        print(elf.header['e_machine'], arch.arch)
        raise Exception('Architecture mismatch')

    for section in elf.iter_sections():
        if not isinstance(section, RelocationSection):
            continue

        if section.name in invalid_relocs:
            print('Invalid VDSO relocation section: %s' %section.name)
            res += 1
            continue

        if section.name not in arch.sections:
            continue

        for reloc in section.iter_relocations():
            if reloc['r_info_type'] in arch.types:
                continue
            res += 1

            symt = elf.get_section(section['sh_link'])
            sym = symt.get_symbol(reloc['r_info_sym'])

            type = describe_reloc_type(reloc['r_info_type'], elf)

            print("Invalid VDSO relocation: %s %s" %(type, sym.name))

    return res

if __name__ == '__main__':
    parser = ArgumentParser(usage = 'usage: %(prog)s arch elf-file',
                            description = 'magic VDSO section checker',
                            prog = 'vdsoreloc')

    parser.add_argument('arch',
                        choices = supported_archs.keys(),
                        help = 'Target architecture')
    parser.add_argument('file', help = 'ELF file to parse')
    args = parser.parse_args()

    with open(args.file, 'rb') as file:
        try:
            res = check_relocations(file, supported_archs[args.arch])
            sys.exit(res)
        except Exception as ex:
            # Do something sensible here
            print(ex)
            sys.exit(1)