cc-wrapper: add support for strictflexarrays1 & strictflexarrays3 hardening flags

adding strictflexarrays1 to pkgsExtraHardening

doc/release-notes/rl-2511.section.md

@@ -15,7 +15,7 @@
 
 <!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->
 
-- Create the first release note entry in this section!
+- New hardening flags, `strictflexarrays1` and `strictflexarrays3` were made available, corresponding to the gcc/clang options `-fstrict-flex-arrays=1` and `-fstrict-flex-arrays=3` respectively.
 
 ## Nixpkgs Library {#sec-nixpkgs-release-25.11-lib}
 

doc/stdenv/stdenv.chapter.md

@@ -1620,6 +1620,22 @@ Adds the `-fPIE` compiler and `-pie` linker options. Position Independent Execut
 Static libraries need to be compiled with `-fPIE` so that executables can link them in with the `-pie` linker option.
 If the libraries lack `-fPIE`, you will get the error `recompile with -fPIE`.
 
+#### `strictflexarrays1` {#strictflexarrays1}
+
+This flag adds the `-fstrict-flex-arrays=1` compiler option, which reduces the cases the compiler treats as "flexible arrays" to those declared with length `[1]`, `[0]` or (the correct) `[]`. This increases the coverage of fortify checks, because such arrays declared as the trailing element of a structure can normally not have their intended length determined by the compiler.
+
+Enabling this flag on packages that still use length declarations of flexible arrays >1 may cause the package to fail to compile citing accesses beyond the bounds of an array or even crash at runtime by detecting an array access as an "overrun". Few projects still use length declarations of flexible arrays >1.
+
+Disabling `strictflexarrays1` implies disablement of `strictflexarrays3`.
+
+#### `strictflexarrays3` {#strictflexarrays3}
+
+This flag adds the `-fstrict-flex-arrays=3` compiler option, which reduces the cases the compiler treats as "flexible arrays" to only those declared with length as (the correct) `[]`. This increases the coverage of fortify checks, because such arrays declared as the trailing element of a structure can normally not have their intended length determined by the compiler.
+
+Enabling this flag on packages that still use non-empty length declarations for flexible arrays may cause the package to fail to compile citing accesses beyond the bounds of an array or even crash at runtime by detecting an array access as an "overrun". Many projects still use such non-empty length declarations for flexible arrays.
+
+Enabling this flag implies enablement of `strictflexarrays1`. Disabling this flag does not imply disablement of `strictflexarrays1`.
+
 #### `shadowstack` {#shadowstack}
 
 Adds the `-fcf-protection=return` compiler option. This enables the Shadow Stack feature supported by some newer processors, which maintains a user-inaccessible copy of the program's stack containing only return-addresses. When returning from a function, the processor compares the return-address value on the two stacks and throws an error if they do not match, considering it a sign of corruption and possible tampering. This should significantly increase the difficulty of ROP attacks.

pkgs/build-support/cc-wrapper/add-hardening.sh

@@ -10,6 +10,7 @@ for flag in ${NIX_HARDENING_ENABLE_@suffixSalt@-}; do
   hardeningEnableMap["$flag"]=1
 done
 
+
 # fortify3 implies fortify enablement - make explicit before
 # we filter unsupported flags because unsupporting fortify3
 # doesn't mean we should unsupport fortify too
@@ -31,8 +32,31 @@ if [[ -n "${hardeningEnableMap[fortify3]-}" ]]; then
   unset -v "hardeningEnableMap['fortify']"
 fi
 
+
+# strictflexarrays3 implies strictflexarrays1 enablement - make explicit before
+# we filter unsupported flags because unsupporting strictflexarrays3
+# doesn't mean we should unsupport strictflexarrays1 too
+if [[ -n "${hardeningEnableMap[strictflexarrays3]-}" ]]; then
+  hardeningEnableMap["strictflexarrays1"]=1
+fi
+
+# Remove unsupported flags.
+for flag in @hardening_unsupported_flags@; do
+  unset -v "hardeningEnableMap[$flag]"
+  # strictflexarrays1 being unsupported implies strictflexarrays3 is unsupported
+  if [[ "$flag" = 'strictflexarrays1' ]] ; then
+    unset -v "hardeningEnableMap['strictflexarrays3']"
+  fi
+done
+
+# now make strictflexarrays1 and strictflexarrays3 mutually exclusive
+if [[ -n "${hardeningEnableMap[strictflexarrays3]-}" ]]; then
+  unset -v "hardeningEnableMap['strictflexarrays1']"
+fi
+
+
 if (( "${NIX_DEBUG:-0}" >= 1 )); then
-  declare -a allHardeningFlags=(fortify fortify3 shadowstack stackprotector stackclashprotection nostrictaliasing pacret pie pic strictoverflow format trivialautovarinit zerocallusedregs)
+  declare -a allHardeningFlags=(fortify fortify3 shadowstack stackprotector stackclashprotection nostrictaliasing pacret strictflexarrays1 strictflexarrays3 pie pic strictoverflow format trivialautovarinit zerocallusedregs)
   declare -A hardeningDisableMap=()
 
   # Determine which flags were effectively disabled so we can report below.
@@ -79,6 +103,14 @@ for flag in "${!hardeningEnableMap[@]}"; do
       if (( "${NIX_DEBUG:-0}" >= 1 )); then echo HARDENING: enabling shadowstack >&2; fi
       hardeningCFlagsBefore+=('-fcf-protection=return')
       ;;
+    strictflexarrays1)
+      if (( "${NIX_DEBUG:-0}" >= 1 )); then echo HARDENING: enabling strictflexarrays1 >&2; fi
+      hardeningCFlagsBefore+=('-fstrict-flex-arrays=1')
+      ;;
+    strictflexarrays3)
+      if (( "${NIX_DEBUG:-0}" >= 1 )); then echo HARDENING: enabling strictflexarrays3 >&2; fi
+      hardeningCFlagsBefore+=('-fstrict-flex-arrays=3')
+      ;;
     pacret)
       if (( "${NIX_DEBUG:-0}" >= 1 )); then echo HARDENING: enabling pacret >&2; fi
       hardeningCFlagsBefore+=('-mbranch-protection=pac-ret')

pkgs/development/compilers/gcc/default.nix

@@ -428,6 +428,10 @@ pipe
             "fortify3"
             "trivialautovarinit"
           ]
+          ++ optionals (!atLeast13) [
+            "strictflexarrays1"
+            "strictflexarrays3"
+          ]
           ++ optional (
             !(targetPlatform.isLinux && targetPlatform.isx86_64 && targetPlatform.libc == "glibc")
           ) "shadowstack"

pkgs/development/compilers/llvm/common/clang/default.nix

@@ -299,6 +299,8 @@ stdenv.mkDerivation (
         ++ lib.optional (
           (lib.versionOlder release_version "15") || !(targetPlatform.isx86_64 || targetPlatform.isAarch64)
         ) "zerocallusedregs"
+        ++ lib.optional (lib.versionOlder release_version "15") "strictflexarrays1"
+        ++ lib.optional (lib.versionOlder release_version "16") "strictflexarrays3"
         ++ (finalAttrs.passthru.hardeningUnsupportedFlags or [ ]);
     };
 

pkgs/stdenv/generic/make-derivation.nix

@@ -33,6 +33,7 @@ let
     optionalAttrs
     optionalString
     optionals
+    pipe
     remove
     splitString
     subtractLists
@@ -159,6 +160,8 @@ let
     "format"
     "fortify"
     "fortify3"
+    "strictflexarrays1"
+    "strictflexarrays3"
     "shadowstack"
     "nostrictaliasing"
     "pacret"
@@ -356,14 +359,18 @@ let
         ) == 0;
       dontAddHostSuffix = attrs ? outputHash && !noNonNativeDeps || !stdenv.hasCC;
 
-      hardeningDisable' =
-        if
-          any (x: x == "fortify") hardeningDisable
-        # disabling fortify implies fortify3 should also be disabled
-        then
-          unique (hardeningDisable ++ [ "fortify3" ])
-        else
-          hardeningDisable;
+      concretizeFlagImplications =
+        flag: impliesFlags: list:
+        if any (x: x == flag) list then (list ++ impliesFlags) else list;
+
+      hardeningDisable' = unique (
+        pipe hardeningDisable [
+          # disabling fortify implies fortify3 should also be disabled
+          (concretizeFlagImplications "fortify" [ "fortify3" ])
+          # disabling strictflexarrays1 implies strictflexarrays3 should also be disabled
+          (concretizeFlagImplications "strictflexarrays1" [ "strictflexarrays3" ])
+        ]
+      );
       defaultHardeningFlags =
         (if stdenv.hasCC then stdenv.cc else { }).defaultHardeningFlags or
         # fallback safe-ish set of flags

pkgs/top-level/variants.nix

@@ -104,6 +104,7 @@ self: super: {
           stdenv = super'.withDefaultHardeningFlags (
             super'.stdenv.cc.defaultHardeningFlags
             ++ [
+              "strictflexarrays1"
               "shadowstack"
               "nostrictaliasing"
               "pacret"