From a277f53f250910d23534a3fc4401840008296f4d Mon Sep 17 00:00:00 2001
From: Ayush Ranjan <ayushranjan@google.com>
Date: Fri, 22 Sep 2023 10:32:39 -0700
Subject: [PATCH] Fix atomicOTrunc implementation in fusefs.

If FUSE connection does not support atomic O_TRUNC and application calls
open(O_TRUNC), FUSE should truncate the file *first* (without a file handle)
and then subsequently open the file with O_TRUNC filtered out.

We were truncating the file after open(). There were 2 issues with it:
1. We were using the inode's file handle, which may be invalid at this point
   because it is donated to the first FD (to save an open(2) RPC I believe)
   and that FD can close it.
2. The truncate() was being done with fhOptions.useFh = true. However,
   sometimes the file handle might be read-only causing truncate to fail.

Fixed both issues by moving the truncate above open() and making it not use any
file handles.

PiperOrigin-RevId: 567658664
---
 pkg/sentry/fsimpl/fuse/inode.go | 39 +++++++++++++++++++--------------
 1 file changed, 22 insertions(+), 17 deletions(-)

diff --git a/pkg/sentry/fsimpl/fuse/inode.go b/pkg/sentry/fsimpl/fuse/inode.go
index 740a568f7..d7c87e227 100644
--- a/pkg/sentry/fsimpl/fuse/inode.go
+++ b/pkg/sentry/fsimpl/fuse/inode.go
@@ -292,6 +292,19 @@ func (i *inode) Open(ctx context.Context, rp *vfs.ResolvingPath, d *kernfs.Dentr
 	// FOPEN_KEEP_CACHE is the default flag for noOpen.
 	fd.OpenFlag = linux.FOPEN_KEEP_CACHE
 
+	truncateRegFile := opts.Flags&linux.O_TRUNC != 0 && i.filemode().FileType() == linux.S_IFREG
+	if truncateRegFile && (i.fh.new || !i.fs.conn.atomicOTrunc) {
+		// If the regular file needs to be truncated, but the connection doesn't
+		// support O_TRUNC or if we are optimizing away the Open RPC, then manually
+		// truncate the file *before* Open. As per libfuse, "If [atomic O_TRUNC is]
+		// disabled, and an application specifies O_TRUNC, fuse first calls
+		// truncate() and then open() with O_TRUNC filtered out.".
+		opts := vfs.SetStatOptions{Stat: linux.Statx{Size: 0, Mask: linux.STATX_SIZE}}
+		if err := i.setAttr(ctx, i.fs.VFSFilesystem(), auth.CredentialsFromContext(ctx), opts, fhOptions{useFh: false}); err != nil {
+			return nil, err
+		}
+	}
+
 	if i.fh.new {
 		fd.OpenFlag = i.fh.flags
 		fd.Fh = i.fh.handle
@@ -300,7 +313,7 @@ func (i *inode) Open(ctx context.Context, rp *vfs.ResolvingPath, d *kernfs.Dentr
 		// opening a directory.
 	} else if !i.fs.conn.noOpen || i.filemode().IsDir() {
 		in := linux.FUSEOpenIn{Flags: opts.Flags & ^uint32(linux.O_CREAT|linux.O_EXCL|linux.O_NOCTTY)}
-		// Truncating with SETATTR instead of O_TRUNC, so clear the flag.
+		// Clear O_TRUNC if the server doesn't support it.
 		if !i.fs.conn.atomicOTrunc {
 			in.Flags &= ^uint32(linux.O_TRUNC)
 		}
@@ -323,6 +336,14 @@ func (i *inode) Open(ctx context.Context, rp *vfs.ResolvingPath, d *kernfs.Dentr
 			}
 			fd.OpenFlag = out.OpenFlag
 			fd.Fh = out.Fh
+			// Open was successful. Update inode's size if atomicOTrunc && O_TRUNC.
+			if truncateRegFile && i.fs.conn.atomicOTrunc {
+				i.fs.conn.mu.Lock()
+				i.attrVersion.Store(i.fs.conn.attributeVersion.Add(1))
+				i.fs.conn.mu.Unlock()
+				i.size.Store(0)
+				i.touchCMtime()
+			}
 		}
 	}
 	if i.filemode().IsDir() {
@@ -338,22 +359,6 @@ func (i *inode) Open(ctx context.Context, rp *vfs.ResolvingPath, d *kernfs.Dentr
 		fd.Nonseekable = true
 	}
 
-	// If atomicOTrunc and O_TRUNC are set, just update the inode's version number
-	// and set its size to 0 since the truncation is handled by the FUSE daemon.
-	// Otherwise send a separate SETATTR to truncate the file size.
-	if opts.Flags&linux.O_TRUNC != 0 && i.filemode().FileType() == linux.S_IFREG {
-		if i.fs.conn.atomicOTrunc {
-			i.fs.conn.mu.Lock()
-			i.attrVersion.Store(i.fs.conn.attributeVersion.Add(1))
-			i.fs.conn.mu.Unlock()
-			i.size.Store(0)
-			i.touchCMtime()
-		} else {
-			opts := vfs.SetStatOptions{Stat: linux.Statx{Size: 0, Mask: linux.STATX_SIZE}}
-			i.setAttr(ctx, i.fs.VFSFilesystem(), auth.CredentialsFromContext(ctx), opts, fhOptions{useFh: true, fh: i.fh.handle})
-		}
-	}
-
 	if err := fd.vfsfd.Init(fdImpl, opts.Flags, rp.Mount(), d.VFSDentry(), fdOptions); err != nil {
 		return nil, err
 	}