Find conflicts with PRoot loader, termux/termux-packages#189

check-elf-conflicts.py

from elftools.elf.elffile import ELFFile

# Expected architecture for ELFs
EXPECTED_MACHINE = "EM_ARM"

# Paths
LOADER_FILE = "loader" # point to "~/termux/proot/src/src/loader/loader"
LINKER_FILE = "linker" # copy from "/system/bin/linker" possibly check with multiple devices

# TODO: glob + try-except NotInterestingElfException
EXEC_FILES = [
    "/data/data/com.termux/files/usr/bin/php",
    "/data/data/com.termux/files/usr/libexec/gcc/arm-linux-androideabi/5.3.0/cc1plus"
]

# From arch.h
EXEC_PIC_ADDRESS   = 0x0f000000
INTERP_PIC_ADDRESS = 0x1f000000

# From execve/enter.c
PAGE_SIZE = 4096
PAGE_MASK = ~(PAGE_SIZE - 1)

class NotInterestingElfException(Exception):
    """
    Exception raised by getElfPages when ELF file appears to be not interesting for us
    (Not for ARM or not executable)
    """
    pass

def getElfPages(path, picOffset):
    """
    Get used pages for given ELF file
    """
    usedPages = set()
    with open(path, "rb") as fi:
        elf = ELFFile(fi)

        if elf.header["e_machine"] != EXPECTED_MACHINE:
            raise NotInterestingElfException("Not matching e_machine %s in %s" % (elf.header["e_machine"], path))

        # Check if this is PIC executable
        # (This is required for use with Android linker anyway,
        #  but static executables can be non-PIC)
        if elf.header["e_type"] == "ET_DYN":
            if picOffset < 0:
                raise Exception("PIC executable but no picOffset specified for %s" % path)
            offset = picOffset
        elif elf.header["e_type"] == "ET_EXEC":
            offset = 0
        else:
            raise NotInterestingElfException("Unexpected e_type %s in %s" % (elf.header["e_type"], path))

        # Iterate over segments
        for seg in elf.iter_segments():
            segHeader = seg.header
            if segHeader["p_type"] == "PT_LOAD":
                # print(segHeader)
                assert segHeader["p_memsz"] >= segHeader["p_filesz"]
                start_address = segHeader["p_vaddr"] & PAGE_MASK
                end_address   = (segHeader["p_vaddr"] + segHeader["p_memsz"] + PAGE_SIZE) & PAGE_MASK
                start_address += offset
                end_address   += offset
                usedPages |= set(range(start_address / PAGE_SIZE, end_address / PAGE_SIZE))

    return frozenset(usedPages)

if __name__ == "__main__":
    linkerPages = getElfPages(LINKER_FILE, INTERP_PIC_ADDRESS)
    loaderPages = getElfPages(LOADER_FILE, -1) # Must be non-PIC

    if not loaderPages.isdisjoint(linkerPages):
        print("Loader overlaps linker")

    for execFile in EXEC_FILES:
        execPages = getElfPages(execFile, EXEC_PIC_ADDRESS)
        if not loaderPages.isdisjoint(execPages):
            print("Loader overlaps executable in " + execFile)
        if not linkerPages.isdisjoint(execPages):
            print("Linker overlaps executable in " + execFile)