From a7aeb10104110581529e7e2399a2166297bb66c4 Mon Sep 17 00:00:00 2001
From: Levi Neely <lkn@darkstar.example.net>
Date: Tue, 3 Mar 2026 16:01:44 +0100
Subject: [PATCH] Add write-permission check in pfs_write and pfs_ftruncate

Cache PSYNC_PERM_MODIFY in openfile->canmodify at open time.
Check canmodify in pfs_write() and pfs_ftruncate() before allowing writes.
Prevents writes after permissions are revoked between open and write.

Fixes #250
---
 pclsync/pfs.c | 13 +++++++++++++
 pclsync/pfs.h |  1 +
 2 files changed, 14 insertions(+)

diff --git a/pclsync/pfs.c b/pclsync/pfs.c
index 3868a08..7dd96b6 100644
--- a/pclsync/pfs.c
+++ b/pclsync/pfs.c
@@ -1324,6 +1324,7 @@ static int pfs_open(const char *path, struct fuse_file_info *fi) {
         of = pfs_create_file(fileid, 0, 0, 0, 1, writeid,
                                   pfs_task_get_ref_locked(folder),
                                   fpath->name, encoder);
+        of->canmodify = (fpath->permissions & PSYNC_PERM_MODIFY) != 0;
         pfs_task_release_folder_tasks_locked(folder);
         psql_unlock();
         pdbg_logf(D_NOTICE, "opening new file %ld %s", (long)fileid, fpath->name);
@@ -1378,6 +1379,7 @@ static int pfs_open(const char *path, struct fuse_file_info *fi) {
       of = pfs_create_file(cr->fileid, fileid, size, hash, 1, writeid,
                                 pfs_task_get_ref_locked(folder),
                                 fpath->name, encoder);
+      of->canmodify = (fpath->permissions & PSYNC_PERM_MODIFY) != 0;
       pfs_task_release_folder_tasks_locked(folder);
       psql_unlock();
       free(fpath);
@@ -1401,6 +1403,7 @@ static int pfs_open(const char *path, struct fuse_file_info *fi) {
                                 lc->datalen, 0, 1, 0,
                                 pfs_task_get_ref_locked(folder),
                                 fpath->name, PSYNC_CRYPTO_INVALID_ENCODER);
+      of->canmodify = (fpath->permissions & PSYNC_PERM_MODIFY) != 0;
       pfs_task_release_folder_tasks_locked(folder);
       psql_unlock();
       free(fpath);
@@ -1481,6 +1484,7 @@ static int pfs_open(const char *path, struct fuse_file_info *fi) {
     of = pfs_create_file(cr->fileid, 0, 0, 0, 1, 0,
                               pfs_task_get_ref_locked(folder), fpath->name,
                               encoder);
+    of->canmodify = (fpath->permissions & PSYNC_PERM_MODIFY) != 0;
     pfs_task_release_folder_tasks_locked(folder);
     psql_unlock();
     of->newfile = 1;
@@ -1506,6 +1510,7 @@ static int pfs_open(const char *path, struct fuse_file_info *fi) {
     of = pfs_create_file(fileid, fileid, size, hash, 0, 0,
                               pfs_task_get_ref_locked(folder), fpath->name,
                               encoder);
+    of->canmodify = (fpath->permissions & PSYNC_PERM_MODIFY) != 0;
     of->origctime = ctime;
     fi->fh = openfile_to_fh(of);
     ret = 0;
@@ -2452,6 +2457,10 @@ static int pfs_write(const char *path, const char *buf, size_t size,
   //  (unsigned long)offset);
   of = fh_to_openfile(fi->fh);
   pfs_lock_file(of);
+  if (!of->canmodify) {
+    pthread_mutex_unlock(&of->mutex);
+    return -EACCES;
+  }
   ret = pfs_check_write_space(of, size, offset);
   if (pdbg_unlikely(ret <= 0))
     return ret;
@@ -3238,6 +3247,10 @@ static int pfs_ftruncate(const char *path, fuse_off_t size,
   pdbg_logf(D_NOTICE, "ftruncate %s %lu", path, (unsigned long)size);
   of = fh_to_openfile(fi->fh);
   pfs_lock_file(of);
+  if (!of->canmodify) {
+    pthread_mutex_unlock(&of->mutex);
+    return -EACCES;
+  }
   ret = pfs_ftruncate_of_locked(of, size);
   pthread_mutex_unlock(&of->mutex);
   return pdbg_returnf(ret, " for ftruncate of %s to %lu", path,
diff --git a/pclsync/pfs.h b/pclsync/pfs.h
index a7c9ac0..f176336 100644
--- a/pclsync/pfs.h
+++ b/pclsync/pfs.h
@@ -108,6 +108,7 @@ typedef struct {
   unsigned char encrypted;
   unsigned char throttle;
   unsigned char staticfile;
+  unsigned char canmodify;
 #if IS_DEBUG
   const char *lockfile;
   const char *lockthread;