Use a tmpfs file for shared anonymous and /dev/zero mmap on VFS2.

This is more consistent with Linux (see comment on MM.NewSharedAnonMappable()).
We don't do the same thing on VFS1 for reasons documented by the updated
comment.

PiperOrigin-RevId: 332514849

2 files changed, 21 insertions, 0 deletions

diff --git a/pkg/sentry/syscalls/linux/sys_mmap.go b/pkg/sentry/syscalls/linux/sys_mmap.go
index 8ab062bca..cd8dfdfa4 100644
--- a/pkg/sentry/syscalls/linux/sys_mmap.go
+++ b/pkg/sentry/syscalls/linux/sys_mmap.go
@@ -100,6 +100,15 @@ func Mmap(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.SyscallC
 		if err := file.ConfigureMMap(t, &opts); err != nil {
 			return 0, nil, err
 		}
+	} else if shared {
+		// Back shared anonymous mappings with a special mappable.
+		opts.Offset = 0
+		m, err := mm.NewSharedAnonMappable(opts.Length, t.Kernel())
+		if err != nil {
+			return 0, nil, err
+		}
+		opts.MappingIdentity = m // transfers ownership of m to opts
+		opts.Mappable = m
 	}
 
 	rv, err := t.MemoryManager().MMap(t, opts)
diff --git a/pkg/sentry/syscalls/linux/vfs2/mmap.go b/pkg/sentry/syscalls/linux/vfs2/mmap.go
index dc05c2994..9d9dbf775 100644
--- a/pkg/sentry/syscalls/linux/vfs2/mmap.go
+++ b/pkg/sentry/syscalls/linux/vfs2/mmap.go
@@ -17,6 +17,7 @@ package vfs2
 import (
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/sentry/arch"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/tmpfs"
 	"gvisor.dev/gvisor/pkg/sentry/kernel"
 	"gvisor.dev/gvisor/pkg/sentry/memmap"
 	"gvisor.dev/gvisor/pkg/syserror"
@@ -85,6 +86,17 @@ func Mmap(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.SyscallC
 		if err := file.ConfigureMMap(t, &opts); err != nil {
 			return 0, nil, err
 		}
+	} else if shared {
+		// Back shared anonymous mappings with an anonymous tmpfs file.
+		opts.Offset = 0
+		file, err := tmpfs.NewZeroFile(t, t.Credentials(), t.Kernel().ShmMount(), opts.Length)
+		if err != nil {
+			return 0, nil, err
+		}
+		defer file.DecRef(t)
+		if err := file.ConfigureMMap(t, &opts); err != nil {
+			return 0, nil, err
+		}
 	}
 
 	rv, err := t.MemoryManager().MMap(t, opts)