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NAME

gcc-local — local modifications to gcc

DESCRIPTION

OpenBSD uses derivatives of gcc(1) versions 2.95.3, 3.3.5, or 4.2.1, depending on machine architecture. In all cases, the software comes with specific modifications for OpenBSD:

• gcc does not search under /usr/local for include files nor for libraries: as a system compiler, it only searches the system paths by default.
• The -pthread option should be used to link threaded code, isolating the program from operating system details.
• On most architectures, trampoline code marks the smallest possible area around the trampoline stub executable using mprotect(2), since the stack area is by default non-executable.
• Trampoline code generation is disabled by default. Code requiring trampolines will not compile without -ftrampolines. The warning flag -Wtrampolines can be used to locate trampoline instances if trampoline generation is re-enabled.
• The -O2 option does not include -fstrict-aliasing, as this option causes issues on some legacy code. -fstrict-aliasing is very unsafe with code that plays tricks with casts, bypassing the already weak type system of C.
• The -O2 option does not include -fstrict-overflow, as this option causes issues on some legacy code. -fstrict-overflow can cause surprising optimizations to occur, possibly deleting security critical overflow checks.
• The -O2 option does not include the -ftree-vrp optimization as it is known to be broken in gcc 4.2.1.
• The option -fno-builtin-<function> was backported from gcc 3.3.5, to gcc 2.95.3, and can be used without having to differentiate between both compilers.
• gcc recognizes the extra format attribute syslog, to better match the definition of syslog(3), and silence erroneous warnings when used with -pedantic.
• Even in 2.95.3, gcc recognizes the attribute __nonnull__, which can be used to mark arguments that can't be NULL. The printf format attribute does not imply __nonnull__ for the format. This allows for correct format checking on the err(3) function family.
• gcc recognizes the extra attribute __sentinel__, which can be used to mark varargs function that need a NULL pointer to mark argument termination, like execl(3). This exposes latent bugs for 64-bit architectures, where a terminating 0 will expand to a 32-bit int, and not a full-fledged 64-bits pointer.
• On some platforms, gcc still uses setjmp(3) / longjmp(3) - style exceptions, and so needs extra fixes beyond the pure 2.95.3 release.
• On a few platforms (mostly a.out), gcc uses a linker wrapper to write stubs that call global constructors and destructors. Those platforms use gcc 2.95.3, and those calls can be traced using -Wl,-trace-ctors-dtors, using syslog_r(3).
• On alpha, -mieee is enabled by default to enable full compliance with the IEEE floating point standard, although the “inexact” flag is not maintained. Additionally, rounding mode is dynamic.
• gcc comes with the “ProPolice” stack protection extension, which is enabled by default. This extension reorders local variable declarations and adds stack consistency checks at run time, in order to detect stack overflows, and will attempt to report the problem in the system logs by calling syslog(3) with a LOG_CRIT priority message: “stack overflow in function XXX”, and abort the faulting process. It can be turned off using the -fno-stack-protector commandline option. Note that the stack protector relies on some support code in libc. Stand-alone programs not linked against libc must either provide their own support bits, or use the -fno-stack-protector option. There is also a -fstack-protector-all option, that turns stack protection code on for all functions, and disables any heuristic that flags some functions as safe. This extended checking has a moderate runtime cost, though.
• gcc recognizes a new flag, -Wbounded, to perform basic checks on functions which accept buffers and sizes. An extra attribute, __bounded__, has been added to mark functions that can be checked this way.
• gcc recognizes a new format attribute, kprintf, to deal with the extra format arguments ‘%b’, ‘%r’, and ‘%z’ used in the OpenBSD kernel.
• gcc does not store its version string in objects. This behavior can be restored with -fident.
• gcc will not move variables initialized with the value zero from the data section to the bss section. The default behaviour of gcc 3.3.5 on other systems is to perform this action, which can be restored for OpenBSD with -fzero-initialized-in-bss.
• gcc does not warn for cast expressions used as lvalues outside of -pedantic.
• gcc 4.2.1 does not warn for pointer arguments passing or assignment with different signedness outside of -pedantic. This can be re-enabled with the -Wpointer-sign flag.
• Even in 2.95.3, gcc recognizes the preprocessor flag -CC that lets comments in macros pass through to the output (except in -traditional mode). This is used to allow annotations in macros for lint(1).
• gcc supports two extra warning options:
  • -Wstack-larger-than-N will report functions using more than N bytes of stack space for their local variables. Stack space used for other purposes (such as register window saving, callee-saved registers, or outbound arguments storage) is not taken into account for this check.
  • -Wvariable-decl will report automatic variable declarations whose size cannot be determined at compile-time.

ATTRIBUTES

The __bounded__ attribute is used to type-check functions whose parameters pass fixed-length buffers and their sizes. The syntax for normal buffers is:

__attribute__ ((__bounded__ ( __buffer__, buffer, length )))

where buffer contains the parameter number (starting from 1) of the pointer to the buffer, and length contains the parameter number of the buffer length argument.

gcc will emit a warning if the length argument is a constant larger than the actual size of the buffer. If the buffer is not a statically declared array of fixed length, no warnings will be generated. Refer to memcpy(3) for an example of a function with this check.

For checking strings, just use __string__ instead of __buffer__:

__attribute__ ((__bounded__ ( __string__, buffer, length )))

In addition to the checks described above, this also tests if the length argument was wrongly derived from a sizeof(void *) operation. strlcpy(3) is a good example of a string function with this check.

Some functions specify the length as two arguments: the number of elements and the size of each element. In this case, use the __size__ attribute:

__attribute__ ((__bounded__ ( __size__, buffer, nmemb, size )))

where buffer contains the parameter number of the pointer to the buffer, nmemb contains the parameter number of the number of members, and size has the parameter number of the size of each element. The type checks performed by __size__ are the same as the __buffer__ attribute. See fread(3) for an example of this type of function.

If a function accepts a buffer parameter and specifies that it has to be of a minimum length, the __minbytes__ attribute can be used:

__attribute__ ((__bounded__ ( __minbytes__, buffer, minsize )))

where buffer contains the parameter number of the pointer to the buffer, and minsize specifies the minimum number of bytes that the buffer should be. ctime_r(3) is an example of this type of function.

If -Wbounded is specified with -Wformat, additional checks are performed on sscanf(3) format strings. The ‘%s’ fields are checked for incorrect bound lengths by checking the size of the buffer associated with the format argument.

SEE ALSO

gcc(1)

http://www.research.ibm.com/trl/projects/security/ssp/

CAVEATS

The -Wbounded flag only works with statically allocated fixed-size buffers. Since it is applied at compile-time, dynamically allocated memory buffers and non-constant arguments are ignored.