From 7fecafe015505c0ebd47780050118ff789a9ae3f Mon Sep 17 00:00:00 2001
From: jvoisin
Date: Thu, 30 Apr 2026 17:57:51 +0200
Subject: Fix a POSIX violation for swab

---
 include/unistd.h           |  2 +-
 tests/Makefile             |  1 +
 tests/test_swab_negative.c | 19 +++++++++++++++++++
 3 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 tests/test_swab_negative.c

diff --git a/include/unistd.h b/include/unistd.h
index a2b3105..a73279b 100644
--- a/include/unistd.h
+++ b/include/unistd.h
@@ -177,7 +177,7 @@ _FORTIFY_FN(swab) void swab(const void * _FORTIFY_POS0 __os,
 	size_t __bs = __bos(__os, 0);
 	size_t __bd = __bos(__od, 0);
 
-	if ((size_t)__n > __bs || (size_t)__n > __bd)
+	if (__n > 0 && ((size_t)__n > __bs || (size_t)__n > __bd))
 		__builtin_trap();
 	return __orig_swab(__os, __od, __n);
 }
diff --git a/tests/Makefile b/tests/Makefile
index fbfd178..9fc8287 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -87,6 +87,7 @@ RUNTIME_TARGETS= \
 	test_strncpy_static_write     \
 	test_swab_dynamic_read        \
 	test_swab_dynamic_write       \
+	test_swab_negative            \
 	test_swab_static_read         \
 	test_ttyname_r_dynamic        \
 	test_ttyname_r_static         \
diff --git a/tests/test_swab_negative.c b/tests/test_swab_negative.c
new file mode 100644
index 0000000..f9c182b
--- /dev/null
+++ b/tests/test_swab_negative.c
@@ -0,0 +1,19 @@
+#include "common.h"
+
+#include <unistd.h>
+
+int main(int argc, char** argv) {
+  char src[8] = "ABCDEFG";
+  char dst[8] = {0};
+
+  /* Positive case: normal swab works */
+  swab(src, dst, 6);
+  puts(dst);
+
+  /* Negative n: POSIX says swab does nothing, must NOT trap */
+  swab(src, dst, -1);
+  swab(src, dst, -100);
+
+  puts(dst);
+  return ret;
+}
-- 
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