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NAME

gcc-local — local modifications to gcc

DESCRIPTION

OpenBSD uses derivatives of gcc(1) version 4.2.1. The software comes with specific modifications for OpenBSD:

• For the C programming language, gcc defaults to the gnu99 standard, not gnu89. The -std option can be used to select a different language standard.
• 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.
• On all architectures where the stack is non-executable, trampoline code marks the smallest possible area around the trampoline stub executable using mprotect(2).
• 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.
• gcc recognizes the extra format attribute syslog, to better match the definition of syslog(3), and silence erroneous warnings when used with -pedantic.
• 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 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 runtime, 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 command line 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 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. There is a -fstack-protector-strong option, similar to -fstack-protector, which includes additional functions to be protected — those that have local array definitions or have references to local frame addresses.

• On amd64, -msave-args can be passed to the compiler to have functions save their register arguments on the stack, while maintaining compatibility with the System 5 AMD64 ABI. This enables tools and debuggers that understand this semantic to trivially generate stack traces that include function arguments.
• On all architectures, gcc generates position-independent executables (PIEs) by default, allowing the system to load the resulting binary at a random location. This behavior can be turned off by passing -fno-pie to the compiler and -nopie to the linker.
• gcc recognizes a new flag which is enabled by default, -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.
• The option -fstack-shuffle will randomize the order of stack variables at compile time, which can be helpful to find bugs.
• gcc will not move variables initialized with the value zero from the data section to the bss section. The default behaviour of gcc 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 does not warn for passing pointer arguments or assignment with different signedness outside of -pedantic. This can be re-enabled with the -Wpointer-sign flag.
• gcc recognizes the preprocessor flag -CC that lets comments in macros pass through to the output (except in -traditional mode).
• The warning option -Wsystem-headers, which makes gcc report warnings in systems headers, is enabled by default.
• gcc supports two extra warning options:
  • -Wframe-larger-than=N (and its non-portable alias -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.
• gcc has backported support for the GCC binary integer constants extension, which was first introduced in gcc 4.3.
• The behavior of -Wshadow in gcc has been altered to behave similarly to gcc 4.8 and not warn about variables or parameters that shadow a global function unless the variable or parameter is of type pointer-to-function.

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.

If a function needs string checking like __string__ but operates on element counts rather than buffer sizes, use __wcstring__:

__attribute__((__bounded__(__wcstring__, buffer, count)))

An example of a string function with this check is wcslcpy(3).

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)

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.