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Example 626 shows code from Version 26 of Squid, a popular opensource Web proxy cache The code adds various parameters to a structure that represents a new server After the primary domain controller and backup domain controller parameters, ParamPDC and ParamBDC, are tested using a DNS lookup, they are copied into the new server structure Because the code does not perform checks on the length of ParamPDC or the other strings it operates on, they can be truncated by the bounded calls to strncpy() If the strings are truncated, they are unlikely to represent valid server names, which contradicts the programmer s expectation because calls to gethostbyname() on these names have already succeeded Although the strings stored in the current element of ServerArray are valid null-terminated strings, they can cause unexpected and dif cult-to-track-down errors elsewhere in the system If attackers create malicious server entries designed to fail when Squid falls back on the server s backup domain controller, they could induce unexpected behavior that is susceptible to other exploits or initiate a denial-of-service attack
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Example 626 These calls to strncpy() from Squid 26 could cause truncation errors
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void AddServer(char *ParamPDC, char *ParamBDC, char *ParamDomain) { if (gethostbyname(ParamPDC) == NULL) { syslog(LOG_ERR, "AddServer: Ignoring host '%s' " "Cannot resolve its address", ParamPDC); return; } if (gethostbyname(ParamBDC) == NULL) { syslog(LOG_USER | LOG_ERR, "AddServer: Ignoring host '%s' " "Cannot resolve its address", ParamBDC); return; } /* NOTE: ServerArray is zeroed in OpenConfigFile() */ assert(Serversqueried < MAXSERVERS); strncpy(ServerArray[Serversqueried]pdc, ParamPDC, NTHOSTLEN-1); strncpy(ServerArray[Serversqueried]bdc, ParamBDC, NTHOSTLEN-1); strncpy(ServerArray[Serversqueried]domain, ParamDomain, NTHOSTLEN-1); Serversqueried++; }
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The code in Example 627 demonstrates a string truncation error that turns into a string termination problem The error is related to the use of the function readlink() Because readlink() does not null-terminate its destination buffer and can return up to the number of bytes speci ed in its third argument, the code in Example 627 falls into the all-too-common trap of manually null-terminating the expanded path (buf, in this case) 1 byte beyond the end of the buffer This off-by-one error might be inconsequential, depending on what is stored in the memory just beyond the buffer, because it will remain effectively null-terminated until the other memory location is overwritten That is, strlen(buf) will return only one larger than the actual size of the buffer, PATH_MAX + 1 in this case However, when buf is subsequently copied into another buffer with the return value of readlink() as the bound passed to strncpy(), the data in buf are truncated and the destination buffer path is left unterminated This off-by-one-error is now likely to cause a serious buffer over ow
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Example 627 A call to strncpy() that could cause a truncation error because of confusion over the behavior of readlink()
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char path[PATH_MAX]; char buf[PATH_MAX]; if(S_ISLNK(stst_mode)) { len = readlink(link, buf, sizeof(path)); buf[len] = '\0'; } strncpy(path, buf, len);
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One of the most important decisions that governs how best to avoid truncation errors is whether your program employs static or dynamic memory allocation Code that manipulates strings can be coded to dynamically reallocate buffers based on the size of the data they operate on, which is attractive because it avoids truncating data in most cases Within the confines of the total memory of the system, programs that typically perform dynamic memory allocation should rarely find it necessary to truncate data
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Programs that employ static memory allocation must choose between two kinds of truncation errors Neither option is as desirable as dynamic reallocation because either can result in the program violating the user s expectations If data exceed the capacity of an existing buffer, the program must either truncate the data to align with the available resources or refuse to perform the operation and demand smaller input The trade-offs between truncation and controlled failure must be weighed The simpler of the two options is to decline to perform the requested operation, which will not likely have any unexpected impact on the rest of the program However, this can result in poor usability if the system frequently receives input that it cannot accommodate Alternatively, if the program truncates the data and continues to execute normally, a variety of errors can ensue These errors typically fall into two camps: The string might no longer convey the same meaning after it is truncated (refer to Example 626) or the string might become unterminated (refer to Example 627) The moral of the story is this: Avoid truncating data silently If the input provided is too large for a given operation, attempt to handle the situation gracefully by dynamically resizing buffers, or decline to perform the operation and indicate to the user what needs to happen for the operation to succeed As a worst-case option, truncate the data and inform the user that truncation has occurred The string functions in the Microsoft Strsafe and Safe CRT libraries make identifying and reporting errors easier Both sets of functions implement runtime checks that cause the functions to fail and invoke customizable error handlers when truncation and other errors occur This improvement over the quiet or silent failure seen with most standard string manipulation functions makes the Microsoft alternatives a signi cant step in the right direction Maintaining the Null Terminator In C, strings depend on proper null termination; without it, their size cannot be determined This dependency is fragile because it relies on the contents of the string to ensure that operations performed on it behave correctly This section outlines common ways that unterminated strings enter
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