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Padding is also referred to as "struct member alignment." Many compilers provide a flag that controls how the members of a structure are packed into memory. Modifying this flag may cause the size of the structures to vary. Most compilers also include a keyword that removes all padding; the resulting structures are referred to as packed structures. Overriding the default behavior is often unwise because it leads to interface compatibility problems (the nominally same struct has its layout interpreted differently in different modules).

Non-Compliant Code Example

This non-compliant code example assumes that the size of {{struct buffer}} is equal to the {{sizeof(size_t) + (sizeof(char) * 50)}}, which may not be the case  \[[Dowd 06|AA. C References#Dowd 06]\]. The size of {{struct buffer}} may actually be larger due to structure padding.

struct buffer {
  size_t size;
  char bufferC[50];
};

/* ... */

void func(struct buffer *buf) {

  /* assuming sizeof(size_t) is 4, sizeof(size_t)+sizeof(char)*50 equals 54 */
  struct buffer *buf_cpy = (struct buffer *)malloc(sizeof(size_t)+(sizeof(char)*50));
  if (buf_cpy == NULL) {
    /* Handle malloc() error */
  }
  /*
   * with padding, sizeof(struct buffer) may be greater than 54, causing
   * some data to be written outside the bounds of the memory allocated
   */
  memcpy(buf_cpy, buf, sizeof(struct buffer));

  /* ... */

  free(buf_cpy);
}

Compliant Solution

Accounting for structure padding prevents these types of errors.

struct buffer {
  size_t size;
  char bufferC[50];
};

/* ... */

void func(struct buffer *buf) {

  struct buffer *buf_cpy = (struct buffer *)malloc(sizeof(struct buffer));
  if (buf_cpy == NULL) {
    /* Handle malloc() error */
  }

  /* ... */

  memcpy(buf_cpy, buf, sizeof(struct buffer));

  /* ... */

  free(buf_cpy);
}

Risk Assessment

Failure to correctly determine the size of a structure can lead to subtle logic errors and incorrect calculations.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[Dowd 06|AA. C References#Dowd 06]\] Chapter 6, "C Language Issues" (Structure Padding 284-287)
\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.7.2.1, "Structure and union specifiers"
\[[Sloss 04|AA. C References#Sloss 04]\] Section 5.7, "Structure Arrangement"

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