From ac76f1928581ed64109634210d0f95e6542b53e8 Mon Sep 17 00:00:00 2001
From: Martin Vychodil <martin.vychodil@espressif.com>
Date: Wed, 19 Aug 2026 11:41:24 +0200
Subject: [PATCH] fix(fatfs): preserve access mode in VFS F_SETFL

F_SETFL was replacing the whole flags word, so fcntl(fd, F_SETFL, O_APPEND)
made F_GETFL report O_RDONLY|O_APPEND. Keep O_ACCMODE and apply only POSIX
status flags.

Co-authored-by: Cursor <cursoragent@cursor.com>
---
 .../flash_wl/main/test_fatfs_flash_wl.c       |  7 +++++
 .../test_fatfs_common/test_fatfs_common.c     | 31 +++++++++++++++++++
 .../test_fatfs_common/test_fatfs_common.h     |  2 ++
 components/fatfs/vfs/vfs_fat.c                |  9 ++++--
 4 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/components/fatfs/test_apps/flash_wl/main/test_fatfs_flash_wl.c b/components/fatfs/test_apps/flash_wl/main/test_fatfs_flash_wl.c
index da182301781..ca9aa172076 100644
--- a/components/fatfs/test_apps/flash_wl/main/test_fatfs_flash_wl.c
+++ b/components/fatfs/test_apps/flash_wl/main/test_fatfs_flash_wl.c
@@ -195,6 +195,13 @@ TEST_CASE("(WL) overwrite and append file", "[fatfs][wear_levelling]")
     test_teardown();
 }
 
+TEST_CASE("(WL) fcntl F_SETFL preserves access mode", "[fatfs][wear_levelling]")
+{
+    test_setup();
+    test_fatfs_fcntl_setfl("/spiflash/fcntl.txt");
+    test_teardown();
+}
+
 TEST_CASE("(WL) can lseek", "[fatfs][wear_levelling]")
 {
     test_setup();
diff --git a/components/fatfs/test_apps/test_fatfs_common/test_fatfs_common.c b/components/fatfs/test_apps/test_fatfs_common/test_fatfs_common.c
index 71b34494b86..f24e5acddd7 100644
--- a/components/fatfs/test_apps/test_fatfs_common/test_fatfs_common.c
+++ b/components/fatfs/test_apps/test_fatfs_common/test_fatfs_common.c
@@ -94,6 +94,37 @@ void test_fatfs_overwrite_append(const char* filename)
     TEST_ASSERT_EQUAL(0, fclose(f_r));
 }
 
+void test_fatfs_fcntl_setfl(const char* filename)
+{
+    unlink(filename);
+    int fd = open(filename, O_RDWR | O_CREAT | O_TRUNC);
+    TEST_ASSERT_NOT_EQUAL(-1, fd);
+
+    int flags = fcntl(fd, F_GETFL);
+    TEST_ASSERT_NOT_EQUAL(-1, flags);
+    TEST_ASSERT_EQUAL(O_RDWR, flags & O_ACCMODE);
+    TEST_ASSERT_EQUAL(0, flags & O_APPEND);
+
+    /* F_SETFL must not clobber O_ACCMODE when arg is only a status flag. */
+    TEST_ASSERT_EQUAL(0, fcntl(fd, F_SETFL, O_APPEND));
+    flags = fcntl(fd, F_GETFL);
+    TEST_ASSERT_EQUAL(O_RDWR, flags & O_ACCMODE);
+    TEST_ASSERT_NOT_EQUAL(0, flags & O_APPEND);
+
+    TEST_ASSERT_EQUAL(4, write(fd, "AAAA", 4));
+    TEST_ASSERT_NOT_EQUAL(-1, lseek(fd, 0, SEEK_SET));
+    TEST_ASSERT_EQUAL(4, write(fd, "BBBB", 4));
+    TEST_ASSERT_EQUAL(0, close(fd));
+
+    char buf[10] = { 0 };
+    FILE *f_r = fopen(filename, "r");
+    TEST_ASSERT_NOT_NULL(f_r);
+    TEST_ASSERT_EQUAL(8, fread(buf, 1, 8, f_r));
+    TEST_ASSERT_EQUAL_STRING_LEN("AAAABBBB", buf, 8);
+    TEST_ASSERT_EQUAL(0, fclose(f_r));
+    unlink(filename);
+}
+
 void test_fatfs_read_file(const char* filename)
 {
     FILE* f = fopen(filename, "r");
diff --git a/components/fatfs/test_apps/test_fatfs_common/test_fatfs_common.h b/components/fatfs/test_apps/test_fatfs_common/test_fatfs_common.h
index 9504fd863b9..1253ec338be 100644
--- a/components/fatfs/test_apps/test_fatfs_common/test_fatfs_common.h
+++ b/components/fatfs/test_apps/test_fatfs_common/test_fatfs_common.h
@@ -33,6 +33,8 @@ void test_fatfs_open_file_with_o_creat_flag(const char* filename);
 
 void test_fatfs_overwrite_append(const char* filename);
 
+void test_fatfs_fcntl_setfl(const char* filename);
+
 void test_fatfs_read_file(const char* filename);
 
 void test_fatfs_read_file_utf_8(const char* filename);
diff --git a/components/fatfs/vfs/vfs_fat.c b/components/fatfs/vfs/vfs_fat.c
index 4595f91c00e..939c4215ac9 100644
--- a/components/fatfs/vfs/vfs_fat.c
+++ b/components/fatfs/vfs/vfs_fat.c
@@ -719,10 +719,15 @@ static int vfs_fat_fcntl(void* ctx, int fd, int cmd, int arg)
         case F_GETFL:
             result = fat_ctx->flags[fd];
             break;
-        case F_SETFL:
-            fat_ctx->flags[fd] = arg;
+        case F_SETFL: {
+            /* POSIX: F_SETFL changes status flags only; access mode is immutable.
+             * FatFS honors O_APPEND on write(). O_NONBLOCK is stored for F_GETFL
+             * round-trip but does not make I/O non-blocking. */
+            const uint32_t setfl_mask = (uint32_t)(O_APPEND | O_NONBLOCK);
+            fat_ctx->flags[fd] = (fat_ctx->flags[fd] & ~setfl_mask) | ((uint32_t)arg & setfl_mask);
             result = 0;
             break;
+        }
         // no-ops:
         case F_SETLK:
         case F_SETLKW: