busybox: fix potential build-time buffer overflow

d8bcd48503 ("fortify-headers: 1.1 -> 3.0.1") (currently on staging)
breaks the musl build by identifying this issue.

pkgs/os-specific/linux/busybox/build-system-buffer-overflow.patch

@@ -0,0 +1,27 @@
+From 3cf1ca7491bd5b6680e80355d76442ae14db681e Mon Sep 17 00:00:00 2001
+From: Alyssa Ross <hi@alyssa.is>
+Date: Sun, 29 Mar 2026 13:18:09 +0200
+Subject: [PATCH] build system: fix potential buffer overflow
+
+This could potentially write one byte past the end of line.
+Identified by fortify-headers.
+---
+ scripts/basic/split-include.c | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/scripts/basic/split-include.c b/scripts/basic/split-include.c
+index 6ef29195e..93011d511 100644
+--- a/scripts/basic/split-include.c
++++ b/scripts/basic/split-include.c
+@@ -195,7 +195,7 @@ int main(int argc, const char * argv [])
+ 	ERROR_EXIT( "find" );
+ 
+     line[0] = '\n';
+-    while (fgets(line+1, buffer_size, fp_find))
++    while (fgets(line+1, buffer_size-1, fp_find))
+     {
+ 	if (strstr(list_target, line) == NULL)
+ 	{
+-- 
+2.53.0
+

pkgs/os-specific/linux/busybox/default.nix

@@ -90,6 +90,8 @@ stdenv.mkDerivation rec {
       excludes = [ "networking/httpd_ratelimit_cgi.c" ]; # New since release.
       hash = "sha256-Msm9sDZrVx7ofunnvnTS73SPKUUpR3Tv5xZ/wBd+rts=";
     })
+    # https://lists.busybox.net/pipermail/busybox/2026-March/092010.html
+    ./build-system-buffer-overflow.patch
   ]
   ++ lib.optional (stdenv.hostPlatform != stdenv.buildPlatform) ./clang-cross.patch;