man/man1/gcc-local.1

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.\" Copyright (c) 2003 Anil Madhavapeddy
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.Dd $Mdocdate: June 10 2017 $
.Dt GCC-LOCAL 1
.Os
.Sh NAME
.Nm gcc-local
.Nd local modifications to gcc
.Sh DESCRIPTION
.Ox
uses derivatives of
.Xr gcc 1
versions 3.3.6 or 4.2.1,
depending on machine architecture.
In all cases,
the software comes with specific modifications for
.Ox :
.Bl -dash
.It
.Nm gcc
does not search under
.Pa /usr/local
for include files nor for libraries:
as a system compiler, it only searches the system paths by default.
.It
On all architectures where the stack is non-executable,
trampoline code marks the smallest possible area around the trampoline stub
executable using
.Xr mprotect 2 .
.It
Trampoline code generation is disabled by default.
Code requiring trampolines will not compile without
.Fl ftrampolines .
The warning flag
.Fl Wtrampolines
can be used to locate trampoline instances if trampoline generation
is re-enabled.
.It
The
.Fl O2
option does not include
.Fl fstrict-aliasing ,
as this option causes issues on some legacy code.
.Fl fstrict-aliasing
is very unsafe with code that plays tricks with casts, bypassing the
already weak type system of C.
.It
The
.Fl O2
option does not include
.Fl fstrict-overflow ,
as this option causes issues on some legacy code.
.Fl fstrict-overflow
can cause surprising optimizations to occur, possibly deleting security
critical overflow checks.
.It
The
.Fl O2
option does not include the
.Fl ftree-vrp
optimization as it is known to be broken in
.Nm gcc 4.2.1 .
.It
.Nm gcc
recognizes the extra format attribute syslog, to better match
the definition of
.Xr syslog 3 ,
and silence erroneous warnings when used with
.Fl pedantic .
.It
.Nm gcc
recognizes the attribute
.Cm __nonnull__ ,
which can be used to mark arguments that can't be
.Dv NULL .
The printf format attribute does not imply
.Cm __nonnull__
for the format.
This allows for correct format checking on the
.Xr err 3
function family.
.It
.Nm gcc
recognizes the extra attribute
.Cm __sentinel__ ,
which can be used to mark varargs function that need a
.Dv NULL
pointer to mark argument termination, like
.Xr 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.
.It
On alpha,
.Fl mieee
is enabled by default to enable full compliance with
the IEEE floating point standard,
although the
.Dq inexact
flag is not maintained.
Additionally, rounding mode is dynamic.
.It
.Nm gcc
comes with the
.Dq 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
.Xr syslog 3
with a
.Dv LOG_CRIT
priority message:
.Dq stack overflow in function XXX ,
and abort the faulting process.
It can be turned off using the
.Fl 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
.Fl fno-stack-protector
option.
.Pp
There is a
.Fl 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
.Fl fstack-protector-strong
option, similar to
.Fl fstack-protector ,
which includes additional functions to be protected \(em those that
have local array definitions
or have references to local frame addresses.
.It
On amd64,
.Fl 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.
.It
On the alpha, amd64, arm, hppa, i386, mips64, powerpc, sh and sparc64
architectures,
.Nm gcc
generates position-independent executables
.Pq PIEs
by default, allowing the system to load the resulting binary
at a random location.
This behavior can be turned off by passing
.Fl fno-pie
to the compiler and
.Fl nopie
to the linker.
.It
.Nm gcc
recognizes a new flag which is enabled by default,
.Fl Wbounded ,
to perform basic checks on functions which accept buffers and sizes.
An extra attribute,
.Cm __bounded__ ,
has been added to mark functions that can be
checked this way.
.It
.Nm gcc
recognizes a new format attribute, kprintf, to deal with the extra format
arguments
.Ql %b ,
.Ql %r ,
and
.Ql %z
used in the
.Ox
kernel.
.It
.Nm gcc
does not store its version string in objects.
This behavior can be restored with
.Fl fident .
.It
The option
.Fl fstack-shuffle
will randomize the order of stack variables at compile time with
.Nm gcc 4.2.1 ,
which can be helpful to find bugs.
This option is silently ignored by
.Nm gcc 3.3.6 .
.It
.Nm gcc
will not move variables initialized with the value zero
from the data section to the bss section.
The default behaviour of
.Nm gcc 3.3.6
and
.Nm gcc 4.2.1
on other systems is to perform this action, which can be restored for
.Ox
with
.Fl fzero-initialized-in-bss .
.It
.Nm gcc
does not warn for cast expressions used as lvalues outside of
.Fl pedantic .
.It
.Nm gcc 4.2.1
does not warn for passing pointer arguments or assignment with
different signedness outside of
.Fl pedantic .
This can be
re-enabled with the
.Fl Wpointer-sign
flag.
.It
.Nm gcc
recognizes the preprocessor flag
.Fl CC
that lets comments in macros pass through to the output (except in
.Fl traditional
mode).
.It
The warning option
.Fl Wsystem-headers ,
which makes
.Nm gcc
report warnings in systems headers,
is enabled by default.
.It
.Nm gcc
supports two extra warning options:
.Bl -item
.It
.Fl Wframe-larger-than= Ns Va N
.Po
and its non-portable alias
.Fl Wstack-larger-than- Ns Va N
.Pc
will report functions using more than
.Va 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.
.It
.Fl Wvariable-decl
will report automatic variable declarations whose size cannot be
determined at compile-time.
.El
.It
.Nm gcc 4.2.1
and
.Nm gcc 3.3.6
have backported support for the GCC binary integer constants extension,
which was first introduced in
.Nm gcc 4.3 .
.It
The behavior of
.Fl Wshadow
in
.Nm gcc 4.2.1
has been altered to behave similarly to
.Nm 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.
.El
.Sh ATTRIBUTES
The
.Cm __bounded__
attribute is used to type-check functions whose parameters pass fixed-length
buffers and their sizes.
The syntax for normal buffers is:
.Pp
.Ic __attribute__ Ns Po Po
.Cm __bounded__ Ns Pq Cm __buffer__ , Va buffer , length
.Pc Pc
.Pp
where
.Fa buffer
contains the parameter number (starting from 1) of the pointer to the buffer,
and
.Fa length
contains the parameter number of the buffer length argument.
.Pp
.Nm 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
.Xr memcpy 3
for an example of a function with this check.
.Pp
For checking strings, just use
.Cm __string__
instead of
.Cm __buffer__ :
.Pp
.Ic __attribute__ Ns Po Po
.Cm __bounded__ Ns Pq Cm __string__ , Va buffer , length
.Pc Pc
.Pp
In addition to the checks described above, this also tests if the
.Va length
argument was wrongly derived from a
.Fn sizeof "void *"
operation.
.Xr strlcpy 3
is a good example of a string function with this check.
.Pp
If a function needs string checking like
.Cm __string__
but operates on element counts rather than buffer sizes, use
.Cm __wcstring__ :
.Pp
.Ic __attribute__ Ns Po Po
.Cm __bounded__ Ns Pq Cm __wcstring__ , Va buffer , count
.Pc Pc
.Pp
An example of a string function with this check is
.Xr wcslcpy 3 .
.Pp
Some functions specify the length as two arguments:
the number of elements and the size of each element.
In this case, use the
.Cm __size__
attribute:
.Pp
.Ic __attribute__ Ns Po Po
.Cm __bounded__ Ns Pq Cm __size__ , Va buffer , nmemb , size
.Pc Pc
.Pp
where
.Va buffer
contains the parameter number of the pointer to the buffer,
.Va nmemb
contains the parameter number of the number of members, and
.Va size
has the parameter number of the size of each element.
The type checks performed by
.Cm __size__
are the same as the
.Cm __buffer__
attribute.
See
.Xr fread 3
for an example of this type of function.
.Pp
If a function accepts a buffer parameter and specifies that it has to be of a
minimum length, the __minbytes__ attribute can be used:
.Pp
.Ic __attribute__ Ns Po Po
.Cm __bounded__ Ns Pq Cm __minbytes__ , Va buffer , minsize
.Pc Pc
.Pp
where
.Va buffer
contains the parameter number of the pointer to the buffer, and
.Va minsize
specifies the minimum number of bytes that the buffer should be.
.Xr ctime_r 3
is an example of this type of function.
.Pp
If
.Fl Wbounded
is specified with
.Fl Wformat ,
additional checks are performed on
.Xr sscanf 3
format strings.
The
.Ql %s
fields are checked for incorrect bound lengths by checking the size of the
buffer associated with the format argument.
.Sh SEE ALSO
.Xr gcc 1
.Sh CAVEATS
The
.Fl 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.