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Hint: people who want to use ubsan can use the ubsan_minimal runtime

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.\" $OpenBSD: clang-local.1,v 1.23 2022/02/18 00:39:18 jca Exp $
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.\" Copyright (c) 2016 Pascal Stumpf <pascal@stumpf.co>
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.Dd $Mdocdate: February 18 2022 $
.Dt CLANG-LOCAL 1
.Os
.Sh NAME
.Nm clang-local
.Nd OpenBSD-specific behavior of LLVM/clang
.Sh DESCRIPTION
On
.Ox ,
the LLVM/clang compiler exhibits the following characteristics:
.Bl -dash
.It
.Nm clang
does not search under
.Pa /usr/local
for include files or libraries:
as a system compiler, it only searches the system paths by default.
.It
.Nm clang
comes with stack protection enabled by default, equivalent to the
.Fl fstack-protector-strong
option on other systems.
The system will report any violation of the stack protector cookie along
with the function name via
.Xr syslog 3
at
.Dv LOG_CRIT
priority.
.It
.Nm clang
will generate PIE code 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 is also turned off when the
.Fl pg
flag is used.
.It
The
.Fl fstrict-aliasing
option is turned off by default unless
.Fl Ofast
has been selected.
.It
.Nm clang
does not store its version string in objects.
There is no option to control this.
.It
The
.Fl p
flag is an alias of
.Fl pg .
.It
.Nm clang
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
The warning option
.Fl Waddress-of-packed-member
is disabled by default.
.It
Color diagnostic messages are disabled by default and can be re-enabled with
.Fl fdiagnostics-color .
.It
The
.Fl fwrapv
option to treat signed integer overflows as defined is enabled by default to
prevent dangerous optimizations which could remove security critical overflow
checks.
.It
The base system includes the ubsan_minimal sanitizer runtime
but not the fully-featured ubsan runtime.
See the documentation for the
.Fl fsanitize-minimal-runtime
flag.
.It
The
.Xr malloc 3 ,
.Xr calloc 3 ,
.Xr realloc 3 ,
.Xr strdup 3 ,
.Xr strndup 3 ,
.Xr valloc 3
and
.Xr free 3
builtins are disabled to prevent undesirable optimizations of calls to
these functions.
.It
.Nm clang
includes a security pass that exchanges some ROP-friendly instructions
with safer alternatives on i386 and amd64.
This can be disabled with the
.Fl fno-fixup-gadgets
option.
.It
.Nm clang
includes the retguard security feature on amd64, arm64, mips64, powerpc
and powerpc64.
This feature can be disabled with the
.Fl fno-ret-protector
or
.Fl fno-stack-protector
options.
.It
.Nm clang
has
.Fl mretpoline
enabled by default on amd64 to protect against branch target injection attacks.
It can be disabled with
.Fl mno-retpoline .
.It
A new warning for
.Cm %n
format specifier usage in
.Xr printf 3
family functions has been added.
.El
.Sh SEE ALSO
.Xr clang 1