From 2f7a7c05512eaa3893dad2c4c178d251f022fb78 Mon Sep 17 00:00:00 2001
From: Koi <helzky@proton.me>
Date: Mon, 2 Nov 2026 00:37:34 -0500
Subject: [PATCH] util: compare file identity in UTIL_isSameFile on Windows

The Windows branch compared names with strcmp, so an output path that
named the input with a different case, a ./ prefix or an absolute path
was not detected and the input was truncated when the output was opened.
Compare the volume serial number and file index instead.

Fixes #4852
---
 programs/util.c    | 54 +++++++++++++++++++++++++++++++++++++---------
 tests/playTests.sh | 11 ++++++++++
 2 files changed, 55 insertions(+), 10 deletions(-)

diff --git a/programs/util.c b/programs/util.c
index 652530b1223..70823633fd2 100644
--- a/programs/util.c
+++ b/programs/util.c
@@ -23,6 +23,7 @@
 #endif /* #ifdef __FreeBSD__ */
 
 #if defined(_WIN32)
+#  include <windows.h>    /* CreateFileA, GetFileInformationByHandle */
 #  include <sys/utime.h>  /* utime */
 #  include <io.h>         /* _chmod */
 #  define ZSTD_USE_UTIMENSAT 0
@@ -381,17 +382,52 @@ int UTIL_compareStr(const void *p1, const void *p2) {
     return strcmp(* (char * const *) p1, * (char * const *) p2);
 }
 
+#if defined(_WIN32)
+/* note : Windows has no inode numbers, but a file's identity is given by
+ *        its volume serial number and file index, which stay the same
+ *        however the path to the file is spelled */
+static int UTIL_isSameFileWin32(const char* fName1, const char* fName2)
+{
+    BY_HANDLE_FILE_INFORMATION file1Info;
+    BY_HANDLE_FILE_INFORMATION file2Info;
+    HANDLE file1Handle;
+    HANDLE file2Handle;
+    int same;
+
+    if (!strcmp(fName1, fName2)) return 1;
+
+    file1Handle = CreateFileA(fName1, FILE_READ_ATTRIBUTES,
+                              FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
+                              NULL, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, NULL);
+    if (file1Handle == INVALID_HANDLE_VALUE) return 0;
+
+    file2Handle = CreateFileA(fName2, FILE_READ_ATTRIBUTES,
+                              FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
+                              NULL, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, NULL);
+    if (file2Handle == INVALID_HANDLE_VALUE) {
+        CloseHandle(file1Handle);
+        return 0;
+    }
+
+    same = GetFileInformationByHandle(file1Handle, &file1Info)
+        && GetFileInformationByHandle(file2Handle, &file2Info)
+        && file1Info.dwVolumeSerialNumber == file2Info.dwVolumeSerialNumber
+        && file1Info.nFileIndexHigh == file2Info.nFileIndexHigh
+        && file1Info.nFileIndexLow == file2Info.nFileIndexLow;
+
+    CloseHandle(file1Handle);
+    CloseHandle(file2Handle);
+    return same;
+}
+#endif
+
 int UTIL_isSameFile(const char* fName1, const char* fName2)
 {
     int ret;
     assert(fName1 != NULL); assert(fName2 != NULL);
     UTIL_TRACE_CALL("UTIL_isSameFile(%s, %s)", fName1, fName2);
 #if defined(_MSC_VER) || defined(_WIN32)
-    /* note : Visual does not support file identification by inode.
-     *        inode does not work on Windows, even with a posix layer, like msys2.
-     *        The following work-around is limited to detecting exact name repetition only,
-     *        aka `filename` is considered different from `subdir/../filename` */
-    ret = !strcmp(fName1, fName2);
+    ret = UTIL_isSameFileWin32(fName1, fName2);
 #else
     {   stat_t file1Stat;
         stat_t file2Stat;
@@ -412,13 +448,11 @@ int UTIL_isSameFileStat(
     assert(fName1 != NULL); assert(fName2 != NULL);
     UTIL_TRACE_CALL("UTIL_isSameFileStat(%s, %s)", fName1, fName2);
 #if defined(_MSC_VER) || defined(_WIN32)
-    /* note : Visual does not support file identification by inode.
-     *        inode does not work on Windows, even with a posix layer, like msys2.
-     *        The following work-around is limited to detecting exact name repetition only,
-     *        aka `filename` is considered different from `subdir/../filename` */
+    /* note : stat_t does not carry a usable file identity on Windows,
+     *        so compare the files themselves */
     (void)file1Stat;
     (void)file2Stat;
-    ret = !strcmp(fName1, fName2);
+    ret = UTIL_isSameFileWin32(fName1, fName2);
 #else
     {
         ret =  (file1Stat->st_dev == file2Stat->st_dev)
diff --git a/tests/playTests.sh b/tests/playTests.sh
index 44bc82b5610..b87e864776a 100755
--- a/tests/playTests.sh
+++ b/tests/playTests.sh
@@ -241,6 +241,17 @@ rm -f tmpCompressed
 zstd tmp -o tmpOut -c > tmpCompressed
 test -f tmpCompressed   # file must be created
 
+println "test : refuse output overwriting input through another spelling"
+cp tmp tmpRef
+zstd -f tmp -o ./tmp && die "should have refused to overwrite input through ./ prefix"
+cmp tmp tmpRef || die "refused ./ output modified the input file"
+if [ "$isWindows" = true ] ; then
+    println "test : refuse output overwriting input through a different case"
+    zstd -f tmp -o TMP && die "should have refused to overwrite input through a different case"
+    cmp tmp tmpRef || die "refused differently-cased output modified the input file"
+fi
+rm -f tmpRef
+
 println "test : forgotten argument"
 cp tmp tmp2
 zstd tmp2 -fo && die "-o must be followed by filename "