From e07b7a8b935c304bf6cf868978038e14e2b4ce9f Mon Sep 17 00:00:00 2001
From: =?utf8?q?cat=20=C3=A6scling?=
 <11325618-aescling@users.noreply.gitlab.com>
Date: Tue, 25 Jul 2023 20:33:13 -0400
Subject: [PATCH] Protect headers we always send from override

The extensibility is not great because execline makes lists difficult.
There are no native lists, not to mention any kind of list.contains
---
 README.md                                     |  4 +++-
 .../binaries/http-print-header-files.execline | 22 ++++++++++---------
 visible-to-httpd/binaries/httpd.execline      |  9 +++++++-
 3 files changed, 23 insertions(+), 12 deletions(-)

diff --git a/README.md b/README.md
index a209adc..a53e7f1 100644
--- a/README.md
+++ b/README.md
@@ -153,7 +153,9 @@ scheme, and a `Content-Type` file, to override any other mechanism for
 determining the Content-Type to send to the client.
 
 Note that a `Content-Type` file in the `headers` folder is always
-ignored.
+ignored. This is also true for the other required or recommended
+HTTP 1.1 headers always sent by httpd.execline, namely
+`Content-Length`, `Date`, and `Last-Modified`.
 
 a former official website for `httpd-execline.eerie.garden`
 used to redirect to this github repository, thanks to
diff --git a/visible-to-httpd/binaries/http-print-header-files.execline b/visible-to-httpd/binaries/http-print-header-files.execline
index 9886081..3b5980a 100755
--- a/visible-to-httpd/binaries/http-print-header-files.execline
+++ b/visible-to-httpd/binaries/http-print-header-files.execline
@@ -6,7 +6,7 @@
 # instances of ${hostname} and ${resource} are substituted with the provided
 # host and resource from the request
 #
-# the script will refuse to override the Content-Type header
+# the script will refuse to override any header already set by httpd.execline
 #
 # execs into self in order to retain an environment variable which marks
 # whether a certain header has been overridden or not
@@ -30,15 +30,20 @@ emptyenv -P
 # performs `basename ${header_file}`
 backtick -E -n header_name {
 	pipeline { s6-echo -n -- ${header_file} }
-	sed "s@.*/([^/]*)@\\1@"
+	pipeline { sed "s@.*/([^/]*)@\\1@" }
+	tr -d " \t\r\n" # paranoid
 }
 
+# ignore protected headers
 ifelse {
-	pipeline { s6-echo -n -- \${header_name} }
-	grep -s -i '^[ 	]*Content-Type[ 	]*$' # paranoid whitespace matching
+	backtick -E -n header_name_lowercase {
+		pipeline { s6-echo -n -- ${header_name} }
+		tr A-Z a-z
+	}
+	s6-test -v httpd_execline_protected_header_${header_name_lowercase}
 }
 {
-	foreground { log.execline "refusing to override Content-Type header" }
+	foreground { log.execline "WARNING: configuration error: ignoring protected header: \""${header_name}\" }
 	http-print-header-directories.execline ${@}
 }
 
@@ -63,12 +68,9 @@ define header_substitution_script
 }
 s6-echo -n -- "
 
-# we’ll strip out `\r`s and `\n`s from filenames and file contents, in
+# we’ll strip out `\r`s and `\n`s from file contents, in
 # case the configuration should ever be made in a mischevious way
-if {
-	pipeline { s6-echo -n -- ${header_name}": " }
-	tr -d "\r\n"
-}
+if {  s6-echo -n -- ${header_name}": " }
 if {
 	backtick -E -n unparsed_header_contents {
 		pipeline { cat ${header_file} }
diff --git a/visible-to-httpd/binaries/httpd.execline b/visible-to-httpd/binaries/httpd.execline
index 275f895..3134664 100755
--- a/visible-to-httpd/binaries/httpd.execline
+++ b/visible-to-httpd/binaries/httpd.execline
@@ -217,7 +217,14 @@ if -X -n -t {
 	# Security Policy; for the latter, consider HTTP 301 redirects
 	#
 	# be warned!! we do not validate these overrides!
-	backtick -n extra_headers { http-get-extra-headers.execline }
+	backtick -n extra_headers {
+		export httpd_execline_protected_header_content-length ""
+		export httpd_execline_protected_header_content-type ""
+		export httpd_execline_protected_header_date ""
+		export httpd_execline_protected_header_last-modified ""
+
+		http-get-extra-headers.execline
+	}
 
 	backtick -D "200 ok" -n status_code_and_message {
 		if { s6-test -r configuration/overrides/${resource}/status_code }
-- 
2.47.3