GCC-LOCAL(1) OpenBSD Reference Manual GCC-LOCAL(1)NAMEgcc-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.
- On OpenBSD, 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.
- On OpenBSD, by default, trampoline code generation is disabled. 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 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 OpenBSD 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.
- On OpenBSD, 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.
- On OpenBSD, 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.
- On OpenBSD, gcc does not warn for cast expressions used as lvalues
outside of -pedantic.
- On OpenBSD, 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).
- On OpenBSD, 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 ALSOgcc(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.
OpenBSD 4.9 July 12, 2010 OpenBSD 4.9
