Lecture 9Aggregate Data Organization
TopicsTopics
Pointers Aggregate Data
Array layout in memoryStructures
February 14, 2012
CSCE 212 Computer Architecture
– 2 –CSCE 212H Spring 2012
OverviewLast TimeLast Time
GDB recursive Lab 2 – Questions due today Test 1 Feb ?? Not Feb 15
NewNew Datalab Pointers Aggregate Data
Array layout in memory Structures
Next Time: Next Time: Test 1 – Feb 23
February 27, Mon. Last day to drop a course or withdraw without a grade of "WF" being recorded (Session C002)
Test 1 Review
– 3 –CSCE 212H Spring 2012
Pointer Code
void s_helper (int x, int *accum){ if (x <= 1) return; else { int z = *accum * x; *accum = z; s_helper (x-1,accum); }}
int sfact(int x){ int val = 1; s_helper(x, &val); return val;}
Top-Level CallRecursive Procedure
Pass pointer to update location
– 4 –CSCE 212H Spring 2012
Temp.Space
%esp
Creating & Initializing Pointer
int sfact(int x){ int val = 1; s_helper(x, &val); return val;}
_sfact:pushl %ebp # Save %ebpmovl %esp,%ebp # Set %ebpsubl $16,%esp # Add 16 bytes movl 8(%ebp),%edx # edx = xmovl $1,-4(%ebp) # val = 1
Using Stack for Local VariableUsing Stack for Local Variable Variable val must be stored on
stackNeed to create pointer to it
Compute pointer as -4(%ebp) Push on stack as second
argument
Initial part of sfact
xRtn adr
Old %ebp %ebp 0 4 8
-4 val = 1
Unused-12 -8
-16
_sfact:pushl %ebp # Save %ebpmovl %esp,%ebp # Set %ebpsubl $16,%esp # Add 16 bytes movl 8(%ebp),%edx # edx = xmovl $1,-4(%ebp) # val = 1
_sfact:pushl %ebp # Save %ebpmovl %esp,%ebp # Set %ebpsubl $16,%esp # Add 16 bytes movl 8(%ebp),%edx # edx = xmovl $1,-4(%ebp) # val = 1
_sfact:pushl %ebp # Save %ebpmovl %esp,%ebp # Set %ebpsubl $16,%esp # Add 16 bytes movl 8(%ebp),%edx # edx = xmovl $1,-4(%ebp) # val = 1
– 5 –CSCE 212H Spring 2012
Passing Pointer
int sfact(int x){ int val = 1; s_helper(x, &val); return val;}
leal -4(%ebp),%eax # Compute &valpushl %eax # Push on stackpushl %edx # Push xcall s_helper # callmovl -4(%ebp),%eax # Return val• • • # Finish
Calling s_helper from sfact
xRtn adr
Old %ebp %ebp 0 4 8
val = 1 -4
Unused-12 -8
-16
%espx&val
Stack at time of call
leal -4(%ebp),%eax # Compute &valpushl %eax # Push on stackpushl %edx # Push xcall s_helper # callmovl -4(%ebp),%eax # Return val• • • # Finish
leal -4(%ebp),%eax # Compute &valpushl %eax # Push on stackpushl %edx # Push xcall s_helper # callmovl -4(%ebp),%eax # Return val• • • # Finish
val =x!
– 6 –CSCE 212H Spring 2012
Using Pointer
• • •movl %ecx,%eax # z = ximull (%edx),%eax # z *= *accummovl %eax,(%edx) # *accum = z• • •
void s_helper (int x, int *accum){ • • • int z = *accum * x; *accum = z; • • •}
Register %ecx holds x Register %edx holds pointer to accum
Use access (%edx) to reference memory
%edxaccum
xx%eax
%ecxaccum*x
accum*x
– 7 –CSCE 212H Spring 2012
Array AllocationBasic PrincipleBasic Principle
T A[L]; Array of data type T and length L Contiguously allocated region of L * sizeof(T) bytes
char string[12];
x x + 12int val[5];
x x + 4 x + 8 x + 12 x + 16 x + 20double a[4];
x + 32x + 24x x + 8 x + 16
char *p[3];
x x + 4 x + 8
– 8 –CSCE 212H Spring 2012
Array AccessBasic PrincipleBasic Principle
T A[L]; Array of data type T and length L Identifier A can be used as a pointer to array element 0
ReferenceReference TypeType ValueValueval[4] int 3val int * xval+1 int * x + 4&val[2] int * x + 8val[5] int ??*(val+1) int 5val + i int * x + 4 i
1 5 2 1 3int val[5];
x x + 4 x + 8 x + 12 x + 16 x + 20
– 9 –CSCE 212H Spring 2012
Array Example
NotesNotes Declaration “zip_dig cmu” equivalent to “int cmu[5]” Example arrays were allocated in successive 20 byte blocks
Not guaranteed to happen in general
typedef int zip_dig[5];
zip_dig cmu = { 1, 5, 2, 1, 3 };zip_dig mit = { 0, 2, 1, 3, 9 };zip_dig ucb = { 9, 4, 7, 2, 0 };
zip_dig cmu; 1 5 2 1 3
16 20 24 28 32 36zip_dig mit; 0 2 1 3 9
36 40 44 48 52 56zip_dig ucb; 9 4 7 2 0
56 60 64 68 72 76
– 10 –CSCE 212H Spring 2012
Array Accessing Example
Memory Reference CodeMemory Reference Code
int get_digit (zip_dig z, int dig){ return z[dig];}
# %edx = z # %eax = dig
movl (%edx,%eax,4),%eax # z[dig]
ComputationComputation Register %edx contains starting
address of array Register %eax contains array
index Desired digit at 4*%eax + %edx Use memory reference (%edx,%eax,4)
– 11 –CSCE 212H Spring 2012
Referencing Examples
Code Does Not Do Any Bounds Checking!Code Does Not Do Any Bounds Checking!
ReferenceReference AddressAddress ValueValue Guaranteed?Guaranteed?mit[3] 36 + 4* 3 = 48 3mit[5] 36 + 4* 5 = 56 9mit[-1] 36 + 4*-1 = 32 3cmu[15] 16 + 4*15 = 76 ?? Out of range behavior implementation-dependent
No guaranteed relative allocation of different arrays
zip_dig cmu; 1 5 2 1 3
16 20 24 28 32 36zip_dig mit; 0 2 1 3 9
36 40 44 48 52 56zip_dig ucb; 9 4 7 2 0
56 60 64 68 72 76
YesYesNoNoNoNoNoNo
– 12 –CSCE 212H Spring 2012
int zd2int(zip_dig z){ int i; int zi = 0; for (i = 0; i < 5; i++) { zi = 10 * zi + z[i]; } return zi;}
Array Loop Example
Original SourceOriginal Source
int zd2int(zip_dig z){ int zi = 0; int *zend = z + 4; do { zi = 10 * zi + *z; z++; } while(z <= zend); return zi;}
Transformed VersionTransformed Version As generated by GCC Eliminate loop variable i Convert array code to
pointer code Express in do-while form
No need to test at entrance
– 13 –CSCE 212H Spring 2012
# %ecx = zxorl %eax,%eax # zi = 0leal 16(%ecx),%ebx # zend = z+4
.L59:leal (%eax,%eax,4),%edx # 5*zimovl (%ecx),%eax # *zaddl $4,%ecx # z++leal (%eax,%edx,2),%eax # zi = *z + 2*(5*zi)cmpl %ebx,%ecx # z : zendjle .L59 # if <= goto loop
Array Loop ImplementationRegistersRegisters
%ecx z%eax zi%ebx zend
ComputationsComputations 10*zi + *z implemented as *z + 2*(zi+4*zi)
z++ increments by 4
int zd2int(zip_dig z){ int zi = 0; int *zend = z + 4; do { zi = 10 * zi + *z; z++; } while(z <= zend); return zi;}
# %ecx = zxorl %eax,%eax # zi = 0leal 16(%ecx),%ebx # zend = z+4
.L59:leal (%eax,%eax,4),%edx # 5*zimovl (%ecx),%eax # *zaddl $4,%ecx # z++leal (%eax,%edx,2),%eax # zi = *z + 2*(5*zi)cmpl %ebx,%ecx # z : zendjle .L59 # if <= goto loop
int zd2int(zip_dig z){ int zi = 0; int *zend = z + 4; do { zi = 10 * zi + *z; z++; } while(z <= zend); return zi;}
# %ecx = zxorl %eax,%eax # zi = 0leal 16(%ecx),%ebx # zend = z+4
.L59:leal (%eax,%eax,4),%edx # 5*zimovl (%ecx),%eax # *zaddl $4,%ecx # z++leal (%eax,%edx,2),%eax # zi = *z + 2*(5*zi)cmpl %ebx,%ecx # z : zendjle .L59 # if <= goto loop
int zd2int(zip_dig z){ int zi = 0; int *zend = z + 4; do { zi = 10 * zi + *z; z++; } while(z <= zend); return zi;}
# %ecx = zxorl %eax,%eax # zi = 0leal 16(%ecx),%ebx # zend = z+4
.L59:leal (%eax,%eax,4),%edx # 5*zimovl (%ecx),%eax # *zaddl $4,%ecx # z++leal (%eax,%edx,2),%eax # zi = *z + 2*(5*zi)cmpl %ebx,%ecx # z : zendjle .L59 # if <= goto loop
int zd2int(zip_dig z){ int zi = 0; int *zend = z + 4; do { zi = 10 * zi + *z; z++; } while(z <= zend); return zi;}
# %ecx = zxorl %eax,%eax # zi = 0leal 16(%ecx),%ebx # zend = z+4
.L59:leal (%eax,%eax,4),%edx # 5*zimovl (%ecx),%eax # *zaddl $4,%ecx # z++leal (%eax,%edx,2),%eax # zi = *z + 2*(5*zi)cmpl %ebx,%ecx # z : zendjle .L59 # if <= goto loop
int zd2int(zip_dig z){ int zi = 0; int *zend = z + 4; do { zi = 10 * zi + *z; z++; } while(z <= zend); return zi;}
– 14 –CSCE 212H Spring 2012
Nested Array Example
Declaration “zip_dig pgh[4]” equivalent to “int pgh[4][5]” Variable pgh denotes array of 4 elements
» Allocated contiguously Each element is an array of 5 int’s
» Allocated contiguously “Row-Major” ordering of all elements guaranteed
#define PCOUNT 4zip_dig pgh[PCOUNT] = {{1, 5, 2, 0, 6}, {1, 5, 2, 1, 3 }, {1, 5, 2, 1, 7 }, {1, 5, 2, 2, 1 }};
zip_digpgh[4];
76 96 116 136 156
1 5 2 0 6 1 5 2 1 3 1 5 2 1 7 1 5 2 2 1
– 15 –CSCE 212H Spring 2012
Nested Array AllocationDeclarationDeclaration
T A[R][C]; Array of data type T R rows, C columns Type T element requires K bytes
Array SizeArray Size R * C * K bytes
ArrangementArrangement Row-Major Ordering
A[0][0] A[0][C-1]
A[R-1][0]
• • •
• • •A[R-1][C-1]
•••
•••
int A[R][C];
A[0][0]
A[0][C-1]
• • •A
[1][0]
A[1][C-1]
• • •A
[R-1][0]
A[R-1][C-1]
• • •• • •
4*R*C Bytes
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• • •
Nested Array Row Access
Row VectorsRow Vectors A[i] is array of C elements Each element of type T Starting address – base address of A + i * C * K
A[i][0]
A[i][C-1]
• • •
A[i]
A[R-1][0]
A[R-1][C-1]
• • •
A[R-1]
• • •
A
A[0][0]
A[0]
[C-1]• • •
A[0]
int A[R][C];
A+i*C*4 A+(R-1)*C*4
– 17 –CSCE 212H Spring 2012
Nested Array Row Access Code
Row VectorRow Vector pgh[index] is array of 5 int’s Starting address pgh+20*index
CodeCode Computes and returns address Compute as pgh + 4*(index+4*index)
int *get_pgh_zip(int index){ return pgh[index];}
# %eax = indexleal (%eax,%eax,4),%eax # 5 * indexleal pgh(,%eax,4),%eax # pgh + (20 * index)
– 18 –CSCE 212H Spring 2012
• • •
Nested Array Element Access Array Elements Array Elements
A[i][j] is element of type T Address of A[i][j] is base-of A + (i * C + j) * K
Base-of A = starting address of array &A[0][0]C = number of columns = number of elements in a rowK = size of individual element
• • •A
[i][j]
A[i][j]
• • •
A[i]
A[R-1][0]
A[R-1][C-1]
• • •
A[R-1]
• • •
A
A[0][0]
A[0]
[C-1]• • •
A[0]
A+i*C*4 A+(R-1)*C*4A+(i*C+j)*4
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Nested Array Element Access CodeArray Elements Array Elements
pgh[index][dig] is int Address:
pgh + 20*index + 4*dig
CodeCode Computes address
pgh + 4*dig + 4*(index+4*index) movl performs memory reference
int get_pgh_digit (int index, int dig){ return pgh[index][dig];}
# %ecx = dig# %eax = indexleal 0(,%ecx,4),%edx # 4*digleal (%eax,%eax,4),%eax # 5*indexmovl pgh(%edx,%eax,4),%eax # *(pgh + 4*dig + 20*index)
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struct rec { int i; int a[3]; int *p;};
Assembly# %eax = val# %edx = rmovl %eax,(%edx) # Mem[r] = val
void set_i(struct rec *r, int val){ r->i = val;}
StructuresConceptConcept
Contiguously-allocated region of memory Refer to members within structure by names Members may be of different types
Accessing Structure MemberAccessing Structure Member
Memory Layout
i a p0 4 16 20
– 21 –CSCE 212H Spring 2012
struct rec { int i; int a[3]; int *p;};
# %ecx = idx# %edx = rleal 0(,%ecx,4),%eax # 4*idxleal 4(%eax,%edx),%eax # r+4*idx+4
int *find_a (struct rec *r, int idx){ return &r->a[idx];}
Generating Pointer to Struct. Member
Generating Pointer to Generating Pointer to Array ElementArray Element Offset of each structure
member determined at compile time
i a p0 4 16
r + 4 + 4*idx
r
– 22 –CSCE 212H Spring 2012
struct rec { int i; int a[3]; int *p;};
# %edx = rmovl (%edx),%ecx # r->ileal 0(,%ecx,4),%eax # 4*(r->i)leal 4(%edx,%eax),%eax # r+4+4*(r->i)movl %eax,16(%edx) # Update r->p
void set_p(struct rec *r){ r->p = &r->a[r->i];}
Structure Referencing (Cont.)C CodeC Code
i a0 4 16
Element i
i a p0 4 16
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Alignment
Aligned DataAligned Data Primitive data type requires K bytes Address must be multiple of K Required on some machines; advised on IA32
treated differently by Linux and Windows!
Motivation for Aligning DataMotivation for Aligning Data Memory accessed by (aligned) double or quad-words
Inefficient to load or store datum that spans quad word boundaries
Virtual memory very tricky when datum spans 2 pages
CompilerCompiler Inserts gaps in structure to ensure correct alignment of
fields
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Specific Cases of AlignmentSize of Primitive Data Type:Size of Primitive Data Type:
1 byte (e.g., char) no restrictions on address
2 bytes (e.g., short) lowest 1 bit of address must be 02
4 bytes (e.g., int, float, char *, etc.) lowest 2 bits of address must be 002
8 bytes (e.g., double) Windows (and most other OS’s & instruction sets):
» lowest 3 bits of address must be 0002 Linux:
» lowest 2 bits of address must be 002
» i.e., treated the same as a 4-byte primitive data type 12 bytes (long double)
Linux:» lowest 2 bits of address must be 002
» i.e., treated the same as a 4-byte primitive data type
– 25 –CSCE 212H Spring 2012
struct S1 { char c; int i[2]; double v;} *p;
Satisfying Alignment with StructuresOffsets Within StructureOffsets Within Structure
Must satisfy element’s alignment requirement
Overall Structure PlacementOverall Structure Placement Each structure has alignment requirement K
Largest alignment of any element Initial address & structure length must be
multiples of K
Example (under Windows):Example (under Windows): K = 8, due to double elementc i[0] i[1] vp+0 p+4 p+8 p+16 p+24
Multiple of 4 Multiple of 8
Multiple of 8 Multiple of 8
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Linux vs. Windows
Windows (including Cygwin):Windows (including Cygwin): K = 8, due to double element
Linux:Linux: K = 4; double treated like a 4-byte data type
struct S1 { char c; int i[2]; double v;} *p;
c i[0] i[1] vp+0 p+4 p+8 p+16 p+24
Multiple of 4 Multiple of 8Multiple of 8 Multiple of 8
c i[0] i[1]p+0 p+4 p+8
Multiple of 4 Multiple of 4Multiple of 4
vp+12 p+20
Multiple of 4
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Overall Alignment Requirementstruct S2 { double x; int i[2]; char c;} *p;
struct S3 { float x[2]; int i[2]; char c;} *p;
p+0 p+12p+8 p+16 Windows: p+24Linux: p+20
ci[0] i[1]x
ci[0] i[1]
p+0 p+12p+8 p+16 p+20
x[0] x[1]
p+4
p must be multiple of: 8 for Windows4 for Linux
p must be multiple of 4 (in either OS)
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Ordering Elements Within Structurestruct S4 { char c1; double v; char c2; int i;} *p;
struct S5 { double v; char c1; char c2; int i;} *p;
c1 ivp+0 p+20p+8 p+16 p+24
c2
c1 ivp+0 p+12p+8 p+16
c2
10 bytes wasted space in Windows
2 bytes wasted space
– 29 –CSCE 212H Spring 2012
Arrays of StructuresPrinciplePrinciple
Allocated by repeating allocation for array type
In general, may nest arrays & structures to arbitrary depth
a[0]a+0
a[1] a[2]a+12 a+24 a+36
• • •
a+12 a+20a+16 a+24
struct S6 { short i; float v; short j;} a[10];
a[1].i a[1].ja[1].v
– 30 –CSCE 212H Spring 2012
Linux Memory LayoutStackStack
Runtime stack (8MB limit)
HeapHeap Dynamically allocated storage When call malloc, calloc, new
DLLsDLLs Dynamically Linked Libraries Library routines (e.g., printf, malloc) Linked into object code when first executed
DataData Statically allocated data E.g., arrays & strings declared in code
TextText Executable machine instructions Read-only
Upper 2 hex digits of address
Red Hatv. 6.2~1920MBmemorylimit
FF
BF
7F
3F
C0
80
40
00
Stack
DLLs
TextData
Heap
Heap
08
– 31 –CSCE 212H Spring 2012
Linux Memory AllocationLinked
BF
7F
3F
80
40
00
Stack
DLLs
TextData
08
Some Heap
BF
7F
3F
80
40
00
Stack
DLLs
TextData
Heap
08
MoreHeap
BF
7F
3F
80
40
00
Stack
DLLs
TextDataHeap
Heap
08
InitiallyBF
7F
3F
80
40
00
Stack
TextData
08
– 32 –CSCE 212H Spring 2012
Text & Stack Example
(gdb) break main(gdb) run Breakpoint 1, 0x804856f in main ()(gdb) print $esp $3 = (void *) 0xbffffc78
MainMain Address 0x804856f should be read 0x0804856f
StackStack Address 0xbffffc78
InitiallyBF
7F
3F
80
40
00
Stack
TextData
08
– 33 –CSCE 212H Spring 2012
Dynamic Linking Example(gdb) print malloc $1 = {<text variable, no debug info>} 0x8048454 <malloc>(gdb) run Program exited normally.(gdb) print malloc $2 = {void *(unsigned int)} 0x40006240 <malloc>
InitiallyInitially Code in text segment that invokes dynamic
linker Address 0x8048454 should be read 0x08048454
FinalFinal Code in DLL region
LinkedBF
7F
3F
80
40
00
Stack
DLLs
TextData
08
– 34 –CSCE 212H Spring 2012
Memory Allocation Example
char big_array[1<<24]; /* 16 MB */char huge_array[1<<28]; /* 256 MB */
int beyond;char *p1, *p2, *p3, *p4;
int useless() { return 0; }
int main(){ p1 = malloc(1 <<28); /* 256 MB */ p2 = malloc(1 << 8); /* 256 B */ p3 = malloc(1 <<28); /* 256 MB */ p4 = malloc(1 << 8); /* 256 B */ /* Some print statements ... */}
– 35 –CSCE 212H Spring 2012
Example Addresses$esp 0xbffffc78p3 0x500b5008p1 0x400b4008Final malloc 0x40006240p4 0x1904a640 p2 0x1904a538beyond 0x1904a524big_array 0x1804a520huge_array 0x0804a510main() 0x0804856fuseless() 0x08048560Initial malloc 0x08048454
BF
7F
3F
80
40
00
Stack
DLLs
TextDataHeap
Heap
08
– 36 –CSCE 212H Spring 2012
C operatorsOperators Associativity() [] -> . left to right! ~ ++ -- + - * & (type) sizeof right to left* / % left to right+ - left to right<< >> left to right< <= > >= left to right== != left to right& left to right^ left to right| left to right&& left to right|| left to right?: right to left= += -= *= /= %= &= ^= != <<= >>= right to left, left to right
Note: Unary +, -, and * have higher precedence than binary forms
– 37 –CSCE 212H Spring 2012
C pointer declarationsint *p p is a pointer to int
int *p[13] p is an array[13] of pointer to int
int *(p[13]) p is an array[13] of pointer to int
int **p p is a pointer to a pointer to an int
int (*p)[13] p is a pointer to an array[13] of int
int *f() f is a function returning a pointer to int
int (*f)() f is a pointer to a function returning int
int (*(*f())[13])() f is a function returning ptr to an array[13] of pointers to functions returning int
int (*(*x[3])())[5] x is an array[3] of pointers to functions returning pointers to array[5] of ints
– 38 –CSCE 212H Spring 2012
Internet Worm and IM WarNovember, 1988November, 1988
Internet Worm attacks thousands of Internet hosts. How did it happen?
July, 1999July, 1999 Microsoft launches MSN Messenger (instant messaging system). Messenger clients can access popular AOL Instant Messaging Service (AIM)
servers
AIMserver
AIMclient
AIMclient
MSNclient
MSNserver
– 39 –CSCE 212H Spring 2012
Internet Worm and IM War (cont.)August 1999August 1999
Mysteriously, Messenger clients can no longer access AIM servers.
Microsoft and AOL begin the IM war:AOL changes server to disallow Messenger clientsMicrosoft makes changes to clients to defeat AOL changes.At least 13 such skirmishes.
How did it happen?
The Internet Worm and AOL/Microsoft War were both The Internet Worm and AOL/Microsoft War were both based on based on stack buffer overflowstack buffer overflow exploits! exploits!
many Unix functions do not check argument sizes.allows target buffers to overflow.
– 40 –CSCE 212H Spring 2012
String Library Code Implementation of Unix function gets
No way to specify limit on number of characters to read
Similar problems with other Unix functionsstrcpy: Copies string of arbitrary lengthscanf, fscanf, sscanf, when given %s conversion specification
/* Get string from stdin */char *gets(char *dest){ int c = getc(); char *p = dest; while (c != EOF && c != '\n') { *p++ = c; c = getc(); } *p = '\0'; return dest;}
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Vulnerable Buffer Code
int main(){ printf("Type a string:"); echo(); return 0;}
/* Echo Line */void echo(){ char buf[4]; /* Way too small! */ gets(buf); puts(buf);}
– 42 –CSCE 212H Spring 2012
Buffer Overflow Executions
unix>./bufdemoType a string:123123
unix>./bufdemoType a string:12345Segmentation Fault
unix>./bufdemoType a string:12345678Segmentation Fault
– 43 –CSCE 212H Spring 2012
Buffer Overflow Stack
echo:pushl %ebp # Save %ebp on stackmovl %esp,%ebpsubl $20,%esp # Allocate space on stackpushl %ebx # Save %ebxaddl $-12,%esp # Allocate space on stackleal -4(%ebp),%ebx # Compute buf as %ebp-4pushl %ebx # Push buf on stackcall gets # Call gets. . .
/* Echo Line */void echo(){ char buf[4]; /* Way too small! */ gets(buf); puts(buf);}
Return AddressSaved %ebp
[3][2][1][0] buf%ebp
StackFrame
for main
StackFrame
for echo
– 44 –CSCE 212H Spring 2012
Buffer Overflow Stack Example
Before call to gets
unix> gdb bufdemo(gdb) break echoBreakpoint 1 at 0x8048583(gdb) runBreakpoint 1, 0x8048583 in echo ()(gdb) print /x *(unsigned *)$ebp$1 = 0xbffff8f8(gdb) print /x *((unsigned *)$ebp + 1)$3 = 0x804864d
8048648: call 804857c <echo> 804864d: mov 0xffffffe8(%ebp),%ebx # Return Point
Return AddressSaved %ebp
[3][2][1][0] buf%ebp
StackFrame
for main
StackFrame
for echo
0xbffff8d8Return Address
Saved %ebp[3][2][1][0] buf
StackFrame
for main
StackFrame
for echo
bf ff f8 f808 04 86 4d
xx xx xx xx
– 45 –CSCE 212H Spring 2012
Buffer Overflow Example #1
Before Call to gets Input = “123”
No Problem
0xbffff8d8Return Address
Saved %ebp[3][2][1][0] buf
StackFrame
for main
StackFrame
for echo
bf ff f8 f808 04 86 4d
00 33 32 31
Return AddressSaved %ebp
[3][2][1][0] buf%ebp
StackFrame
for main
StackFrame
for echo
– 46 –CSCE 212H Spring 2012
Buffer Overflow Stack Example #2Input = “12345”
8048592: push %ebx 8048593: call 80483e4 <_init+0x50> # gets 8048598: mov 0xffffffe8(%ebp),%ebx 804859b: mov %ebp,%esp 804859d: pop %ebp # %ebp gets set to invalid value 804859e: ret
echo code:
0xbffff8d8Return Address
Saved %ebp[3][2][1][0] buf
StackFrame
for main
StackFrame
for echo
bf ff 00 3508 04 86 4d
34 33 32 31
Return AddressSaved %ebp
[3][2][1][0] buf%ebp
StackFrame
for main
StackFrame
for echo
Saved value of %ebp set to 0xbfff0035
Bad news when later attempt to restore %ebp
– 47 –CSCE 212H Spring 2012
Buffer Overflow Stack Example #3
Input = “12345678”
Return AddressSaved %ebp
[3][2][1][0] buf%ebp
StackFrame
for main
StackFrame
for echo
8048648: call 804857c <echo> 804864d: mov 0xffffffe8(%ebp),%ebx # Return Point
0xbffff8d8Return Address
Saved %ebp[3][2][1][0] buf
StackFrame
for main
StackFrame
for echo
38 37 36 3508 04 86 00
34 33 32 31
Invalid address
No longer pointing to desired return point
%ebp and return address corrupted
– 48 –CSCE 212H Spring 2012
Malicious Use of Buffer Overflow
Input string contains byte representation of executable code Overwrite return address with address of buffer When bar() executes ret, will jump to exploit code
void bar() { char buf[64]; gets(buf); ... }
void foo(){ bar(); ...}
Stack after call to gets()
B
returnaddress
A
foo stack frame
bar stack frame
B
exploitcode
pad
data written
bygets()