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diff --git a/share/man/man7/mitigations.7 b/man/man7/mitigations.7
--- a/share/man/man7/mitigations.7
+++ b/man/man7/mitigations.7
@@ -28,7 +28,7 @@
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
-.Dd July 23, 2024
+.Dd January 24, 2025
.Dt MITIGATIONS 7
.Os
.Sh NAME
@@ -317,6 +317,15 @@
This effectively provides finer-grained protection than SSP for some class of
function and system calls, along with some protection for buffers allocated as
part of the program data.
+.Pp
+Additionally building with
+.Dq FORTIFY_SOURCE
+considers a trailing array as a flexible array only if declared as [], [0], or [1],
+instead of assuming that every trailing array is a flexible array.
+This enables additional bounds checks at runtime for the above mentioned
+functions and cases,
+and performs some additional build-time checks which abort the compilation when
+an out-of-bounds access is detected.
.\"
.Ss Supervisor mode memory protection
Certain processors include features that prevent unintended access to memory
diff --git a/share/mk/bsd.sys.mk b/mk/bsd.sys.mk
--- a/share/mk/bsd.sys.mk
+++ b/mk/bsd.sys.mk
@@ -308,8 +308,11 @@
CFLAGS+= ${SSP_CFLAGS}
.endif # SSP
.if ${FORTIFY_SOURCE} > 0
-CFLAGS+= -D_FORTIFY_SOURCE=${FORTIFY_SOURCE}
-CXXFLAGS+= -D_FORTIFY_SOURCE=${FORTIFY_SOURCE}
+# Ideally we want -fstrict-flex-arrays=3, but even =2 shows issues.
+# Code should be converted to use C99 "array[]" instead of GCC "array[0]"
+# or non-standard "array[1]" to be able to switch to =3.
+CFLAGS+= -D_FORTIFY_SOURCE=${FORTIFY_SOURCE} -fstrict-flex-arrays=1
+CXXFLAGS+= -D_FORTIFY_SOURCE=${FORTIFY_SOURCE} -D_GLIBCXX_ASSERTIONS -fstrict-flex-arrays=1
.endif
# Additional flags passed in CFLAGS and CXXFLAGS when MK_DEBUG_FILES is

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