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Full Discussion: Inexplicable buffer crash
Top Forums Programming Inexplicable buffer crash Post 302965820 by Corona688 on Wednesday 3rd of February 2016 11:37:07 AM
Old 02-03-2016
Inexplicable buffer crash

I am building a wrapper around fgets, and fighting crashes for no reason I can explain.

I have stripped it down as far as I can and it still crashes.

Code:
#include <stdio.h>
#include <stdlib.h>

typedef struct {
        char buf[4096];
        FILE *fp;
        int type;
} wrap;

wrap *wrap_open(const char *file) {
        wrap *wrap=malloc(sizeof(wrap));
        wrap->fp=fopen(file, "r");
        wrap->type=fgetc(wrap->fp);
        ungetc(wrap->type, wrap->fp);
        return(wrap);
}

int wrap_close(wrap *d) {
        if(d == NULL) return(-1);
        if(d->fp)       fclose(d->fp);
        free(d);
        return(0);
}

int wrap_read(wrap *d) {
        if(fgets(d->buf, 100, d->fp) == NULL)
        {
                perror("EOF");
                return(0);
        }

        return(1);
}

int main(int argc, char *argv[]) {
        int n;
        wrap *d;

        if(argc < 2) return(1);

        d=wrap_open(argv[1]);
        if(d == NULL) return(2);

        for(n=0; n<10; n++)
                if(!wrap_read(d)) break;

        wrap_close(d);
        return(0);
}

The first inexplicable thing happens during fgets(), which fails for no good reason. perror prints "EOF: Success". The loop quits early due to this mysterious failure, and when wrap_free() is called, the program dies with "*** Error in `./a.out': munmap_chunk(): invalid pointer: 0x0000000000c8f030 ***", where c8f030 was a valid pointer to wrap.

This does not happen at all when buf[] doesn't reside inside the structure. Everything is happy in that case. Clearly an overflow is happening, but I have no idea why or where.

I am not overflowing the bounds of the array. I checked that, and 100 bytes can't be overflowing 4K anyway.

I am definitely allocating enough memory. sizeof(wrap) is larger than 4096 bytes.

The values of wrap and wrap->fp are not being corrupted.

No mysterious properties of ungetc are crashing it. It still dies without it.

What obvious thing have I missed?
 

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FGETS(3)						   BSD Library Functions Manual 						  FGETS(3)

NAME
fgets, gets -- get a line from a stream LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <stdio.h> char * fgets(char *restrict s, int n, FILE *restrict stream); char * gets(char *s); DESCRIPTION
The fgets() function reads at most one less than the number of characters specified by n from the given stream and stores them in the string s. Reading stops when a newline character is found, at end-of-file or error. The newline, if any, is retained. If any characters are read and there is no error, a '' character is appended to end the string. The gets() function is equivalent to fgets() with an infinite n and a stream of stdin, except that the newline character (if any) is not stored in the string. It is the caller's responsibility to ensure that the input line, if any, is sufficiently short to fit in the string. RETURN VALUES
Upon successful completion, fgets() and gets() return a pointer to the string. If end-of-file occurs before any characters are read, they return NULL and the buffer contents remain unchanged. If an error occurs, they return NULL and the buffer contents are indeterminate. The fgets() and gets() functions do not distinguish between end-of-file and error; callers must use feof(3) and ferror(3) to determine which occurred. ERRORS
[EBADF] The given stream is not a readable stream. The function fgets() may also fail and set errno for any of the errors specified for the routines fflush(3), fstat(2), read(2), or malloc(3). The function gets() may also fail and set errno for any of the errors specified for the routine getchar(3). SECURITY CONSIDERATIONS
The gets() function cannot be used securely. Because of its lack of bounds checking, and the inability for the calling program to reliably determine the length of the next incoming line, the use of this function enables malicious users to arbitrarily change a running program's functionality through a buffer overflow attack. It is strongly suggested that the fgets() function be used in all cases. (See the FSA.) SEE ALSO
feof(3), ferror(3), fgetln(3), fgetws(3) STANDARDS
The functions fgets() and gets() conform to ISO/IEC 9899:1999 (``ISO C99''). BSD
June 4, 1993 BSD
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