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#1 2012-10-10 12:30:57

dougwoodrow
Member
From: UK
Registered: 2012-10-04
Posts: 36

StrLCopy() not adding the null terminator?

Unless I'm doing something very silly, there is a bug in the SynopseRTL version of StrLCopy().

The Borland version always adds a null terminator to Dest, the Synopse version does not in my tests.

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#2 2012-10-10 15:50:09

dougwoodrow
Member
From: UK
Registered: 2012-10-04
Posts: 36

Re: StrLCopy() not adding the null terminator?

I don't understand why you scan the Source for nulls first; is there any reason why the function can't be as simple as this? :

function SynopseStrLCopy(Dest: PChar; const Source: PChar; MaxLen: Cardinal): PChar;
asm
    or edx,edx                // check Source
    jz @z                     // abort if nil
    or eax,eax                // check Dest
    jz @z                     // abort if nil
    mov byte ptr [eax+ecx],0  // null-terminate Dest
    push eax                  // save Dest
    xchg eax,edx              // swap Source & Dest for Move()
    call Move                 // do the move
    pop eax                   // restore Dest
@z:
end;

(Also, the original Delphi7 version doesn't appear to check for nils).

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#3 2012-10-10 15:56:21

ab
Administrator
From: France
Registered: 2010-06-21
Posts: 14,649
Website

Re: StrLCopy() not adding the null terminator?

AFAIR I re-used some optimized version around.
Searching for null is for safety: you copy up to MaxLen bytes, but you copy only source chars len, with a MaxLen value set to the destination buffer.
Goal is to avoid any potential destination buffer overflow, for security reasons.

Yours sounds like a nice alternative, but does not work as expected: you will append a #0 at MaxLen position.

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#4 2012-10-10 16:19:58

dougwoodrow
Member
From: UK
Registered: 2012-10-04
Posts: 36

Re: StrLCopy() not adding the null terminator?

The original Delphi7 code produces an access violation if either the source or the destination is nil, so I think this is the logical equivalent:

function StrLCopy(Dest: PChar; const Source: PChar; MaxLen: Cardinal): PChar;
asm
    mov byte ptr [eax+ecx],0  // null-terminate Dest
    push eax                  // save Dest
    xchg eax,edx              // swap Source & Dest for Move()
    call Move                 // do the move
    pop eax                   // restore Dest
end;

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#5 2012-10-10 16:27:06

dougwoodrow
Member
From: UK
Registered: 2012-10-04
Posts: 36

Re: StrLCopy() not adding the null terminator?

ab wrote:

Searching for null is for safety: you copy up to MaxLen bytes, but you copy only source chars len, with a MaxLen value set to the destination buffer.
Goal is to avoid any potential destination buffer overflow, for security reasons.

That sounds reasonable, but if so why not add similar checks in the Move routine?


Yours sounds like a nice alternative, but does not work as expected: you will append a #0 at MaxLen position.

I'm not sure what you mean; in my tests it behaves exactly the same as the standard Delphi version (including when MaxLen=0).

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#6 2012-10-10 16:58:10

dougwoodrow
Member
From: UK
Registered: 2012-10-04
Posts: 36

Re: StrLCopy() not adding the null terminator?

Simple test unit:

Unit1.pas:

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls, ExtCtrls;

type
  TForm1 = class(TForm)
    btnTestBorland: TButton;
    edtBorland: TLabeledEdit;
    edtSynopse: TLabeledEdit;
    edtSize: TLabeledEdit;
    edtSource: TLabeledEdit;
    btnTestSynopse: TButton;
    btnTestNew: TButton;
    edtNew: TLabeledEdit;
    procedure edtSizeChange(Sender: TObject);
    procedure btnTestBorlandClick(Sender: TObject);
    procedure btnTestSynopseClick(Sender: TObject);
    procedure btnTestNewClick(Sender: TObject);
  private
    { Private declarations }
    Size: Cardinal;
    Buffer: array[0..128] of Char;
    procedure PrepareBuffer;
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

function SynopseStrLCopy(Dest: PChar; const Source: PChar; MaxLen: Cardinal): PChar;
asm // faster version by AB
    or edx,edx
    jz @z
    push eax
    push ebx
    xchg eax,edx
    mov ebx,ecx
    xor ecx,ecx
@1: cmp byte ptr [eax+ecx],0
    lea ecx,ecx+1 // copy last #0
    je @s
    cmp ecx,ebx
    jb @1
@s: pop ebx
    call Move
    pop eax
@z:
end;

function NewStrLCopy(Dest: PChar; const Source: PChar; MaxLen: Cardinal): PChar;
asm
    or edx,edx                // check Source
    jz @z                     // abort if nil
    or eax,eax                // check Dest
    jz @z                     // abort if nil
    mov byte ptr [eax+ecx],0  // null-terminate Dest
    push eax                  // save Dest
    xchg eax,edx              // swap Source & Dest for Move()
    call Move                 // do the move
    pop eax                   // restore Dest
@z:
end;


function BorlandStrLCopy(Dest: PChar; const Source: PChar; MaxLen: Cardinal): PChar;
// original Borland version
asm
        PUSH    EDI
        PUSH    ESI
        PUSH    EBX
        MOV     ESI,EAX
        MOV     EDI,EDX
        MOV     EBX,ECX
        XOR     AL,AL
        TEST    ECX,ECX
        JZ      @@1
        REPNE   SCASB
        JNE     @@1
        INC     ECX
@@1:    SUB     EBX,ECX
        MOV     EDI,ESI
        MOV     ESI,EDX
        MOV     EDX,EDI
        MOV     ECX,EBX
        SHR     ECX,2
        REP     MOVSD
        MOV     ECX,EBX
        AND     ECX,3
        REP     MOVSB
        STOSB
        MOV     EAX,EDX
        POP     EBX
        POP     ESI
        POP     EDI
end;

procedure TForm1.edtSizeChange(Sender: TObject);
begin
  Size := StrToInt(edtSize.Text);
end;

procedure TForm1.PrepareBuffer;
var
  i: Integer;
begin
  for i := 0 to Size-1 do
    Buffer[i] := Char(i);
  Buffer[Size] := '!';
end;

procedure TForm1.btnTestBorlandClick(Sender: TObject);
begin
  PrepareBuffer;
  BorlandStrLCopy(Buffer, PChar(edtSource.Text), Size);
  edtBorland.Text := Buffer;
end;

procedure TForm1.btnTestSynopseClick(Sender: TObject);
begin
  PrepareBuffer;
  SynopseStrLCopy(Buffer, PChar(edtSource.Text), Size);
  edtSynopse.Text := Buffer;
end;

procedure TForm1.btnTestNewClick(Sender: TObject);
begin
  PrepareBuffer;
  NewStrLCopy(Buffer, PChar(edtSource.Text), Size);
  edtNew.Text := Buffer;
end;

end.

Unit1.dfm:

object Form1: TForm1
  Left = 585
  Top = 173
  Width = 374
  Height = 296
  Caption = 'Form1'
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'MS Sans Serif'
  Font.Style = []
  OldCreateOrder = False
  PixelsPerInch = 96
  TextHeight = 13
  object btnTestBorland: TButton
    Left = 56
    Top = 96
    Width = 89
    Height = 25
    Caption = 'btnTestBorland'
    TabOrder = 0
    OnClick = btnTestBorlandClick
  end
  object edtBorland: TLabeledEdit
    Left = 160
    Top = 96
    Width = 121
    Height = 21
    EditLabel.Width = 51
    EditLabel.Height = 13
    EditLabel.Caption = 'edtBorland'
    TabOrder = 1
  end
  object edtSynopse: TLabeledEdit
    Left = 160
    Top = 144
    Width = 121
    Height = 21
    EditLabel.Width = 56
    EditLabel.Height = 13
    EditLabel.Caption = 'edtSynopse'
    TabOrder = 2
  end
  object edtSize: TLabeledEdit
    Left = 216
    Top = 40
    Width = 65
    Height = 21
    EditLabel.Width = 20
    EditLabel.Height = 13
    EditLabel.Caption = 'Size'
    TabOrder = 3
    OnChange = edtSizeChange
  end
  object edtSource: TLabeledEdit
    Left = 56
    Top = 40
    Width = 129
    Height = 21
    EditLabel.Width = 49
    EditLabel.Height = 13
    EditLabel.Caption = 'edtSource'
    TabOrder = 4
    Text = 'ABCDEFGHIJKLMNOP'
  end
  object btnTestSynopse: TButton
    Left = 56
    Top = 144
    Width = 89
    Height = 25
    Caption = 'btnTestSynopse'
    TabOrder = 5
    OnClick = btnTestSynopseClick
  end
  object btnTestNew: TButton
    Left = 56
    Top = 192
    Width = 89
    Height = 25
    Caption = 'btnTestNew'
    TabOrder = 6
    OnClick = btnTestNewClick
  end
  object edtNew: TLabeledEdit
    Left = 160
    Top = 192
    Width = 121
    Height = 21
    EditLabel.Width = 37
    EditLabel.Height = 13
    EditLabel.Caption = 'edtNew'
    TabOrder = 7
  end
end

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#7 2012-10-11 18:53:44

dougwoodrow
Member
From: UK
Registered: 2012-10-04
Posts: 36

Re: StrLCopy() not adding the null terminator?

What puzzles me is why this hasn't been mentioned before, because any standard database application using TStringField.SetAsString (i.e. StrField.AsString := 'x') will highlight the problem.

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#8 2012-10-14 18:40:02

Starkis
Member
From: Up in the space
Registered: 2011-01-16
Posts: 27

Re: StrLCopy() not adding the null terminator?

guess, that code is part of the fastcode library (correct if that's not so). By the way, is all the fastcode library code part of the enhanced RTL?

--- we no need no water, let the ... burn ---

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