FAILED: patch "[PATCH] bootconfig: fix NULL-pointer arithmetic in" failed to apply to 6.18-stable tree

FAILED: patch "[PATCH] bootconfig: fix NULL-pointer arithmetic in" failed to apply to 6.18-stable tree

[gregkh]

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x dec4d8118c179b3d12bca7e609054c6011c4f2ce
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026072111-dizzy-spur-667f@gregkh' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From dec4d8118c179b3d12bca7e609054c6011c4f2ce Mon Sep 17 00:00:00 2001
From: Breno Leitao <leitao@debian.org>
Date: Fri, 26 Jun 2026 05:50:10 -0700
Subject: [PATCH] bootconfig: fix NULL-pointer arithmetic in
 xbc_snprint_cmdline()

xbc_snprint_cmdline() is meant to be called twice: first with
buf=NULL, size=0 to probe the rendered length, then with a real
buffer to fill it (the standard snprintf() two-pass pattern). The
probe call makes the function compute "buf + size" (NULL + 0) and,
on every iteration, advance "buf += ret" from that NULL base and
pass the result back into snprintf().

Pointer arithmetic on a NULL pointer is undefined behavior. It is
harmless in the in-kernel callers today, but the follow-up patches
run this same code in the userspace tools/bootconfig parser at kernel
build time, where host UBSan / FORTIFY_SOURCE abort the build.

Track a running written length (size_t) instead of mutating @buf, and
only form "buf + len" when @buf is non-NULL. snprintf(NULL, 0, ...)
is itself well defined and returns the would-be length, so the
two-pass "probe then fill" usage returns identical byte counts.

Link: https://lore.kernel.org/all/20260626-bootconfig_using_tools-v7-1-24ab72139c29@debian.org/

Fixes: 51887d03aca1 ("bootconfig: init: Allow admin to use bootconfig for kernel command line")
Cc: stable@vger.kernel.org
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

diff --git a/lib/bootconfig.c b/lib/bootconfig.c
index f445b7703fdd..2ed9ee3dc81c 100644
--- a/lib/bootconfig.c
+++ b/lib/bootconfig.c
@@ -427,10 +427,18 @@ static char xbc_namebuf[XBC_KEYLEN_MAX] __initdata;
 int __init xbc_snprint_cmdline(char *buf, size_t size, struct xbc_node *root)
 {
 	struct xbc_node *knode, *vnode;
-	char *end = buf + size;
 	const char *val, *q;
+	size_t len = 0;
 	int ret;
 
+	/*
+	 * Track the running written length rather than advancing @buf, so we
+	 * never form "buf + size" or "buf += ret" while @buf is NULL (the
+	 * size-probe call passes buf=NULL, size=0). NULL pointer arithmetic
+	 * is undefined behavior and trips host UBSan / FORTIFY_SOURCE when
+	 * this renderer runs at kernel build time. snprintf(NULL, 0, ...)
+	 * itself is well defined and returns the would-be length.
+	 */
 	xbc_node_for_each_key_value(root, knode, val) {
 		ret = xbc_node_compose_key_after(root, knode,
 					xbc_namebuf, XBC_KEYLEN_MAX);
@@ -439,10 +447,11 @@ int __init xbc_snprint_cmdline(char *buf, size_t size, struct xbc_node *root)
 
 		vnode = xbc_node_get_child(knode);
 		if (!vnode) {
-			ret = snprintf(buf, rest(buf, end), "%s ", xbc_namebuf);
+			ret = snprintf(buf ? buf + len : NULL, rest(len, size),
+				       "%s ", xbc_namebuf);
 			if (ret < 0)
 				return ret;
-			buf += ret;
+			len += ret;
 			continue;
 		}
 		xbc_array_for_each_value(vnode, val) {
@@ -452,15 +461,15 @@ int __init xbc_snprint_cmdline(char *buf, size_t size, struct xbc_node *root)
 			 * whitespace.
 			 */
 			q = strpbrk(val, " \t\r\n") ? "\"" : "";
-			ret = snprintf(buf, rest(buf, end), "%s=%s%s%s ",
-				       xbc_namebuf, q, val, q);
+			ret = snprintf(buf ? buf + len : NULL, rest(len, size),
+				       "%s=%s%s%s ", xbc_namebuf, q, val, q);
 			if (ret < 0)
 				return ret;
-			buf += ret;
+			len += ret;
 		}
 	}
 
-	return buf - (end - size);
+	return len;
 }
 #undef rest
 

[PATCH 6.18.y 1/2] bootconfig: move xbc_snprint_cmdline() to lib/bootconfig.c

[Sasha Levin]

From: Breno Leitao <leitao@debian.org>

[ Upstream commit 5a643e4623238e14b03d75ca0d4eda0645720cee ]

Move xbc_snprint_cmdline() from init/main.c to lib/bootconfig.c so the
function (and its xbc_namebuf scratch buffer) becomes part of the shared
parser library. tools/bootconfig already compiles lib/bootconfig.c
directly, which lets a follow-up patch reuse the same renderer in the
userspace tool to convert a bootconfig file into a flat cmdline string
at build time.

No functional change.

Link: https://lore.kernel.org/all/20260508-bootconfig_using_tools-v1-1-1132219aa773@debian.org/

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Stable-dep-of: dec4d8118c17 ("bootconfig: fix NULL-pointer arithmetic in xbc_snprint_cmdline()")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/linux/bootconfig.h |  3 ++
 init/main.c                | 45 ------------------------------
 lib/bootconfig.c           | 56 ++++++++++++++++++++++++++++++++++++++
 3 files changed, 59 insertions(+), 45 deletions(-)

diff --git a/include/linux/bootconfig.h b/include/linux/bootconfig.h
index 25df9260d206da..1f6e9c54a2a5ef 100644
--- a/include/linux/bootconfig.h
+++ b/include/linux/bootconfig.h
@@ -265,6 +265,9 @@ static inline struct xbc_node * __init xbc_node_get_subkey(struct xbc_node *node
 int __init xbc_node_compose_key_after(struct xbc_node *root,
 			struct xbc_node *node, char *buf, size_t size);
 
+/* Render key/value pairs under @root as a flat cmdline string */
+int __init xbc_snprint_cmdline(char *buf, size_t size, struct xbc_node *root);
+
 /**
  * xbc_node_compose_key() - Compose full key string of the XBC node
  * @node: An XBC node.
diff --git a/init/main.c b/init/main.c
index 048a6253824297..e3ef1eb34aa3e5 100644
--- a/init/main.c
+++ b/init/main.c
@@ -320,51 +320,6 @@ static void * __init get_boot_config_from_initrd(size_t *_size)
 
 #ifdef CONFIG_BOOT_CONFIG
 
-static char xbc_namebuf[XBC_KEYLEN_MAX] __initdata;
-
-#define rest(dst, end) ((end) > (dst) ? (end) - (dst) : 0)
-
-static int __init xbc_snprint_cmdline(char *buf, size_t size,
-				      struct xbc_node *root)
-{
-	struct xbc_node *knode, *vnode;
-	char *end = buf + size;
-	const char *val, *q;
-	int ret;
-
-	xbc_node_for_each_key_value(root, knode, val) {
-		ret = xbc_node_compose_key_after(root, knode,
-					xbc_namebuf, XBC_KEYLEN_MAX);
-		if (ret < 0)
-			return ret;
-
-		vnode = xbc_node_get_child(knode);
-		if (!vnode) {
-			ret = snprintf(buf, rest(buf, end), "%s ", xbc_namebuf);
-			if (ret < 0)
-				return ret;
-			buf += ret;
-			continue;
-		}
-		xbc_array_for_each_value(vnode, val) {
-			/*
-			 * For prettier and more readable /proc/cmdline, only
-			 * quote the value when necessary, i.e. when it contains
-			 * whitespace.
-			 */
-			q = strpbrk(val, " \t\r\n") ? "\"" : "";
-			ret = snprintf(buf, rest(buf, end), "%s=%s%s%s ",
-				       xbc_namebuf, q, val, q);
-			if (ret < 0)
-				return ret;
-			buf += ret;
-		}
-	}
-
-	return buf - (end - size);
-}
-#undef rest
-
 /* Make an extra command line under given key word */
 static char * __init xbc_make_cmdline(const char *key)
 {
diff --git a/lib/bootconfig.c b/lib/bootconfig.c
index 5d3802eba52a3b..c149ee8eb55dfc 100644
--- a/lib/bootconfig.c
+++ b/lib/bootconfig.c
@@ -405,6 +405,62 @@ const char * __init xbc_node_find_next_key_value(struct xbc_node *root,
 		return "";	/* No value key */
 }
 
+static char xbc_namebuf[XBC_KEYLEN_MAX] __initdata;
+
+#define rest(dst, end) ((end) > (dst) ? (end) - (dst) : 0)
+
+/**
+ * xbc_snprint_cmdline() - Render bootconfig keys under @root as a cmdline string
+ * @buf: Destination buffer (may be NULL when @size is 0 to query the length)
+ * @size: Size of @buf in bytes
+ * @root: Subtree root whose key=value pairs should be rendered
+ *
+ * Walk all key/value pairs under @root and emit them as a space-separated
+ * cmdline string into @buf. Values containing whitespace are quoted with
+ * double quotes. Returns the number of bytes that would be written if @buf
+ * were large enough (matching snprintf semantics), or a negative errno on
+ * failure.
+ */
+int __init xbc_snprint_cmdline(char *buf, size_t size, struct xbc_node *root)
+{
+	struct xbc_node *knode, *vnode;
+	char *end = buf + size;
+	const char *val, *q;
+	int ret;
+
+	xbc_node_for_each_key_value(root, knode, val) {
+		ret = xbc_node_compose_key_after(root, knode,
+					xbc_namebuf, XBC_KEYLEN_MAX);
+		if (ret < 0)
+			return ret;
+
+		vnode = xbc_node_get_child(knode);
+		if (!vnode) {
+			ret = snprintf(buf, rest(buf, end), "%s ", xbc_namebuf);
+			if (ret < 0)
+				return ret;
+			buf += ret;
+			continue;
+		}
+		xbc_array_for_each_value(vnode, val) {
+			/*
+			 * For prettier and more readable /proc/cmdline, only
+			 * quote the value when necessary, i.e. when it contains
+			 * whitespace.
+			 */
+			q = strpbrk(val, " \t\r\n") ? "\"" : "";
+			ret = snprintf(buf, rest(buf, end), "%s=%s%s%s ",
+				       xbc_namebuf, q, val, q);
+			if (ret < 0)
+				return ret;
+			buf += ret;
+		}
+	}
+
+	return buf - (end - size);
+}
+#undef rest
+
 /* XBC parse and tree build */
 
 static int __init xbc_init_node(struct xbc_node *node, char *data, uint32_t flag)
-- 
2.53.0

[PATCH 6.18.y 2/2] bootconfig: fix NULL-pointer arithmetic in xbc_snprint_cmdline()

[Sasha Levin]

From: Breno Leitao <leitao@debian.org>

[ Upstream commit dec4d8118c179b3d12bca7e609054c6011c4f2ce ]

xbc_snprint_cmdline() is meant to be called twice: first with
buf=NULL, size=0 to probe the rendered length, then with a real
buffer to fill it (the standard snprintf() two-pass pattern). The
probe call makes the function compute "buf + size" (NULL + 0) and,
on every iteration, advance "buf += ret" from that NULL base and
pass the result back into snprintf().

Pointer arithmetic on a NULL pointer is undefined behavior. It is
harmless in the in-kernel callers today, but the follow-up patches
run this same code in the userspace tools/bootconfig parser at kernel
build time, where host UBSan / FORTIFY_SOURCE abort the build.

Track a running written length (size_t) instead of mutating @buf, and
only form "buf + len" when @buf is non-NULL. snprintf(NULL, 0, ...)
is itself well defined and returns the would-be length, so the
two-pass "probe then fill" usage returns identical byte counts.

Link: https://lore.kernel.org/all/20260626-bootconfig_using_tools-v7-1-24ab72139c29@debian.org/

Fixes: 51887d03aca1 ("bootconfig: init: Allow admin to use bootconfig for kernel command line")
Cc: stable@vger.kernel.org
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 lib/bootconfig.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/lib/bootconfig.c b/lib/bootconfig.c
index c149ee8eb55dfc..b4e8e17e29155e 100644
--- a/lib/bootconfig.c
+++ b/lib/bootconfig.c
@@ -424,10 +424,18 @@ static char xbc_namebuf[XBC_KEYLEN_MAX] __initdata;
 int __init xbc_snprint_cmdline(char *buf, size_t size, struct xbc_node *root)
 {
 	struct xbc_node *knode, *vnode;
-	char *end = buf + size;
 	const char *val, *q;
+	size_t len = 0;
 	int ret;
 
+	/*
+	 * Track the running written length rather than advancing @buf, so we
+	 * never form "buf + size" or "buf += ret" while @buf is NULL (the
+	 * size-probe call passes buf=NULL, size=0). NULL pointer arithmetic
+	 * is undefined behavior and trips host UBSan / FORTIFY_SOURCE when
+	 * this renderer runs at kernel build time. snprintf(NULL, 0, ...)
+	 * itself is well defined and returns the would-be length.
+	 */
 	xbc_node_for_each_key_value(root, knode, val) {
 		ret = xbc_node_compose_key_after(root, knode,
 					xbc_namebuf, XBC_KEYLEN_MAX);
@@ -436,10 +444,11 @@ int __init xbc_snprint_cmdline(char *buf, size_t size, struct xbc_node *root)
 
 		vnode = xbc_node_get_child(knode);
 		if (!vnode) {
-			ret = snprintf(buf, rest(buf, end), "%s ", xbc_namebuf);
+			ret = snprintf(buf ? buf + len : NULL, rest(len, size),
+				       "%s ", xbc_namebuf);
 			if (ret < 0)
 				return ret;
-			buf += ret;
+			len += ret;
 			continue;
 		}
 		xbc_array_for_each_value(vnode, val) {
@@ -449,15 +458,15 @@ int __init xbc_snprint_cmdline(char *buf, size_t size, struct xbc_node *root)
 			 * whitespace.
 			 */
 			q = strpbrk(val, " \t\r\n") ? "\"" : "";
-			ret = snprintf(buf, rest(buf, end), "%s=%s%s%s ",
-				       xbc_namebuf, q, val, q);
+			ret = snprintf(buf ? buf + len : NULL, rest(len, size),
+				       "%s=%s%s%s ", xbc_namebuf, q, val, q);
 			if (ret < 0)
 				return ret;
-			buf += ret;
+			len += ret;
 		}
 	}
 
-	return buf - (end - size);
+	return len;
 }
 #undef rest
 
-- 
2.53.0