archive: expand only a single %(describe) per archive

Every %(describe) placeholder in $Format:...$ strings in files with the
attribute export-subst is expanded by calling git describe.  This can
potentially result in a lot of such calls per archive.  That's OK for
local repositories under control of the user of git archive, but could
be a problem for hosted repositories.

Expand only a single %(describe) placeholder per archive for now to
avoid denial-of-service attacks.  We can make this limit configurable
later if needed, but let's start out simple.

Reported-by: Jeff King <peff@peff.net>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitattributes.txt

@@ -1174,7 +1174,8 @@ tag then no replacement will be done.  The placeholders are the same
 as those for the option `--pretty=format:` of linkgit:git-log[1],
 except that they need to be wrapped like this: `$Format:PLACEHOLDERS$`
 in the file.  E.g. the string `$Format:%H$` will be replaced by the
-commit hash.
+commit hash.  However, only one `%(describe)` placeholder is expanded
+per archive to avoid denial-of-service attacks.
 
 
 Packing objects

archive.c

@@ -37,13 +37,10 @@ void init_archivers(void)
 
 static void format_subst(const struct commit *commit,
 			 const char *src, size_t len,
-			 struct strbuf *buf)
+			 struct strbuf *buf, struct pretty_print_context *ctx)
 {
 	char *to_free = NULL;
 	struct strbuf fmt = STRBUF_INIT;
-	struct pretty_print_context ctx = {0};
-	ctx.date_mode.type = DATE_NORMAL;
-	ctx.abbrev = DEFAULT_ABBREV;
 
 	if (src == buf->buf)
 		to_free = strbuf_detach(buf, NULL);
@@ -61,7 +58,7 @@ static void format_subst(const struct commit *commit,
 		strbuf_add(&fmt, b + 8, c - b - 8);
 
 		strbuf_add(buf, src, b - src);
-		format_commit_message(commit, fmt.buf, buf, &ctx);
+		format_commit_message(commit, fmt.buf, buf, ctx);
 		len -= c + 1 - src;
 		src  = c + 1;
 	}
@@ -94,7 +91,7 @@ static void *object_file_to_archive(const struct archiver_args *args,
 		strbuf_attach(&buf, buffer, *sizep, *sizep + 1);
 		convert_to_working_tree(args->repo->index, path, buf.buf, buf.len, &buf, &meta);
 		if (commit)
-			format_subst(commit, buf.buf, buf.len, &buf);
+			format_subst(commit, buf.buf, buf.len, &buf, args->pretty_ctx);
 		buffer = strbuf_detach(&buf, &size);
 		*sizep = size;
 	}
@@ -633,12 +630,19 @@ int write_archive(int argc, const char **argv, const char *prefix,
 		  const char *name_hint, int remote)
 {
 	const struct archiver *ar = NULL;
+	struct pretty_print_describe_status describe_status = {0};
+	struct pretty_print_context ctx = {0};
 	struct archiver_args args;
 	int rc;
 
 	git_config_get_bool("uploadarchive.allowunreachable", &remote_allow_unreachable);
 	git_config(git_default_config, NULL);
 
+	describe_status.max_invocations = 1;
+	ctx.date_mode.type = DATE_NORMAL;
+	ctx.abbrev = DEFAULT_ABBREV;
+	ctx.describe_status = &describe_status;
+	args.pretty_ctx = &ctx;
 	args.repo = repo;
 	args.prefix = prefix;
 	string_list_init(&args.extra_files, 1);

archive.h

@@ -5,6 +5,7 @@
 #include "pathspec.h"
 
 struct repository;
+struct pretty_print_context;
 
 struct archiver_args {
 	struct repository *repo;
@@ -22,6 +23,7 @@ struct archiver_args {
 	unsigned int convert : 1;
 	int compression_level;
 	struct string_list extra_files;
+	struct pretty_print_context *pretty_ctx;
 };
 
 /* main api */

pretty.c

@@ -1247,6 +1247,14 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 		struct child_process cmd = CHILD_PROCESS_INIT;
 		struct strbuf out = STRBUF_INIT;
 		struct strbuf err = STRBUF_INIT;
+		struct pretty_print_describe_status *describe_status;
+
+		describe_status = c->pretty_ctx->describe_status;
+		if (describe_status) {
+			if (!describe_status->max_invocations)
+				return 0;
+			describe_status->max_invocations--;
+		}
 
 		cmd.git_cmd = 1;
 		strvec_push(&cmd.args, "describe");

pretty.h

@@ -23,6 +23,10 @@ enum cmit_fmt {
 	CMIT_FMT_UNSPECIFIED
 };
 
+struct pretty_print_describe_status {
+	unsigned int max_invocations;
+};
+
 struct pretty_print_context {
 	/*
 	 * Callers should tweak these to change the behavior of pp_* functions.
@@ -44,6 +48,7 @@ struct pretty_print_context {
 	int color;
 	struct ident_split *from_ident;
 	unsigned encode_email_headers:1;
+	struct pretty_print_describe_status *describe_status;
 
 	/*
 	 * Fields below here are manipulated internally by pp_* functions and

t/t5001-archive-attr.sh

@@ -128,4 +128,18 @@ test_expect_success 'export-subst' '
 	test_cmp substfile2 archive/substfile2
 '
 
+test_expect_success 'export-subst expands %(describe) once' '
+	echo "\$Format:%(describe)\$" >substfile3 &&
+	echo "\$Format:%(describe)\$" >>substfile3 &&
+	echo "\$Format:%(describe)${LF}%(describe)\$" >substfile4 &&
+	git add substfile[34] &&
+	git commit -m export-subst-describe &&
+	git tag -m export-subst-describe export-subst-describe &&
+	git archive HEAD >archive-describe.tar &&
+	extract_tar_to_dir archive-describe &&
+	desc=$(git describe) &&
+	grep -F "$desc" archive-describe/substfile[34] >substituted &&
+	test_line_count = 1 substituted
+'
+
 test_done