Windows2000/private/ntos/rtl/checksum.c

/*++
Copyright (c) 1991  Microsoft Corporation

Module Name:
    imagedir.c

Abstract:
    The module contains the code to translate an image directory type to the address of the data for that entry.

Author:
    Steve Wood (stevewo) 18-Aug-1989

Environment:
    User Mode or Kernel Mode
--*/

#include "ntrtlp.h"


// Define forward referenced prootypes.
USHORT ChkSum(ULONG PartialSum, PUSHORT Source, ULONG Length);

#if defined(ALLOC_PRAGMA) && defined(NTOS_KERNEL_RUNTIME)
#if !defined(_IA64_)
#pragma alloc_text(PAGE,ChkSum)
#endif // !defined(_IA64_)
#pragma alloc_text(PAGE,LdrVerifyMappedImageMatchesChecksum)
#endif

#if !defined(_IA64_)
USHORT ChkSum(ULONG PartialSum, PUSHORT Source, ULONG Length)
/*++
Routine Description:
    Compute a partial checksum on a portion of an imagefile.
Arguments:
    PartialSum - Supplies the initial checksum value.
    Sources - Supplies a pointer to the array of words for which the checksum is computed.
    Length - Supplies the length of the array in words.
Return Value:
    The computed checksum value is returned as the function value.
--*/
{
    RTL_PAGED_CODE();

    // Compute the word wise checksum allowing carries to occur into the high order half of the checksum longword.
    while (Length--) {
        PartialSum += *Source++;
        PartialSum = (PartialSum >> 16) + (PartialSum & 0xffff);
    }

    // Fold final carry into a single word result and return the resultant value.
    return (USHORT)(((PartialSum >> 16) + PartialSum) & 0xffff);
}
#endif // !defined(_IA64_)


BOOLEAN LdrVerifyMappedImageMatchesChecksum (IN PVOID BaseAddress, IN ULONG FileLength)
/*++
Routine Description:
    This functions computes the checksum of an image mapped as a data file.
Arguments:
    BaseAddress - Supplies a pointer to the base of the mapped file.
    FileLength - Supplies the length of the file in bytes.
Return Value:
    TRUE - The checksum stored in the image matches the checksum of the data.
    FALSE - The checksum in the image is not correct.
--*/
{
    PUSHORT AdjustSum;
    PIMAGE_NT_HEADERS NtHeaders;
    USHORT PartialSum;
    ULONG HeaderSum;
    ULONG CheckSum;

    RTL_PAGED_CODE();

    // Compute the checksum of the file and zero the header checksum value.
    HeaderSum = 0;
    PartialSum = ChkSum(0, (PUSHORT)BaseAddress, (FileLength + 1) >> 1);

    // If the file is an image file, then subtract the two checksum words
    // in the optional header from the computed checksum before adding
    // the file length, and set the value of the header checksum.
    NtHeaders = RtlImageNtHeader(BaseAddress);
    if (NtHeaders != NULL) {
        HeaderSum = NtHeaders->OptionalHeader.CheckSum;

#ifndef NTOS_KERNEL_RUNTIME
        // On Nt 3.1 and 3.5, we allowed printer drivers with 0 checksums into
        // csrss unintentionally. This means that we must allow this forever.
        // I don't want to allow this for kernel mode drivers, so I will only
        // allow 0 checksums of the high order bit is clear ?
        if ( HeaderSum == 0 ) {
            return TRUE;
        }
#endif // NTOS_KERNEL_RUNTIME

        AdjustSum = (PUSHORT)(&NtHeaders->OptionalHeader.CheckSum);
        PartialSum -= (PartialSum < AdjustSum[0]);
        PartialSum -= AdjustSum[0];
        PartialSum -= (PartialSum < AdjustSum[1]);
        PartialSum -= AdjustSum[1];
    } else {
        PartialSum = 0;
        HeaderSum = FileLength;
    }

    // Compute the final checksum value as the sum of the paritial checksum and the file length.
    CheckSum = (ULONG)PartialSum + FileLength;
    return (CheckSum == HeaderSum);
}