[PATCH v2 2/3] fs: create helper file_user_path() for user displayed mapped file path

Amir Goldstein
2023-10-07T08:44:59Z

Overlayfs uses backing files with "fake" overlayfs f_path and "real"
underlying f_inode, in order to use underlying inode aops for mapped
files and to display the overlayfs path in /proc/<pid>/maps.

In preparation for storing the overlayfs "fake" path instead of the
underlying "real" path in struct backing_file, define a noop helper
file_user_path() that returns f_path for now.

Use the new helper in procfs and kernel logs whenever a path of a
mapped file is displayed to users.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
arch/arc/kernel/troubleshoot.c | 3 ++-
fs/proc/base.c | 2 +-
fs/proc/nommu.c | 2 +-
fs/proc/task_mmu.c | 4 ++--
fs/proc/task_nommu.c | 2 +-
include/linux/fs.h | 14 ++++++++++++++
kernel/trace/trace_output.c | 2 +-
7 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/arch/arc/kernel/troubleshoot.c b/arch/arc/kernel/troubleshoot.c
index d5b3ed2c58f5..097887e4a9b7 100644
--- a/arch/arc/kernel/troubleshoot.c
+++ b/arch/arc/kernel/troubleshoot.c
@@ -93,7 +93,8 @@ static void show_faulting_vma(unsigned long address)
char *nm = "?";

if (vma->vm_file) {
- nm = file_path(vma->vm_file, buf, ARC_PATH_MAX-1);
+ nm = d_path(file_user_path(vma->vm_file), buf,
+ ARC_PATH_MAX-1);
if (IS_ERR(nm))
nm = "?";
}
diff --git a/fs/proc/base.c b/fs/proc/base.c
index ffd54617c354..20695c928ee6 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2218,7 +2218,7 @@ static int map_files_get_link(struct dentry *dentry, struct path *path)
rc = -ENOENT;
vma = find_exact_vma(mm, vm_start, vm_end);
if (vma && vma->vm_file) {
- *path = vma->vm_file->f_path;
+ *path = *file_user_path(vma->vm_file);
path_get(path);
rc = 0;
}
diff --git a/fs/proc/nommu.c b/fs/proc/nommu.c
index 4d3493579458..c6e7ebc63756 100644
--- a/fs/proc/nommu.c
+++ b/fs/proc/nommu.c
@@ -58,7 +58,7 @@ static int nommu_region_show(struct seq_file *m, struct vm_region *region)

if (file) {
seq_pad(m, ' ');
- seq_file_path(m, file, "");
+ seq_path(m, file_user_path(file), "");
}

seq_putc(m, '\n');
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 3dd5be96691b..1593940ca01e 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -296,7 +296,7 @@ show_map_vma(struct seq_file *m, struct vm_area_struct *vma)
if (anon_name)
seq_printf(m, "[anon_shmem:%s]", anon_name->name);
else
- seq_file_path(m, file, "\n");
+ seq_path(m, file_user_path(file), "\n");
goto done;
}

@@ -1967,7 +1967,7 @@ static int show_numa_map(struct seq_file *m, void *v)

if (file) {
seq_puts(m, " file=");
- seq_file_path(m, file, "\n\t= ");
+ seq_path(m, file_user_path(file), "\n\t= ");
} else if (vma_is_initial_heap(vma)) {
seq_puts(m, " heap");
} else if (vma_is_initial_stack(vma)) {
diff --git a/fs/proc/task_nommu.c b/fs/proc/task_nommu.c
index 7cebd397cc26..bce674533000 100644
--- a/fs/proc/task_nommu.c
+++ b/fs/proc/task_nommu.c
@@ -157,7 +157,7 @@ static int nommu_vma_show(struct seq_file *m, struct vm_area_struct *vma)

if (file) {
seq_pad(m, ' ');
- seq_file_path(m, file, "");
+ seq_path(m, file_user_path(file), "");
} else if (mm && vma_is_initial_stack(vma)) {
seq_pad(m, ' ');
seq_puts(m, "[stack]");
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 7f7e7d2efbeb..a8e4e1cac48e 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2472,6 +2472,20 @@ static inline const struct path *file_real_path(struct file *f)
return &f->f_path;
}

+/*
+ * file_user_path - get the path to display for memory mapped file
+ *
+ * When mmapping a file on a stackable filesystem (e.g., overlayfs), the file
+ * stored in ->vm_file is a backing file whose f_inode is on the underlying
+ * filesystem. When the mapped file path is displayed to user (e.g. via
+ * /proc/<pid>/maps), this helper should be used to get the path to display
+ * to the user, which is the path of the fd that user has requested to map.
+ */
+static inline const struct path *file_user_path(struct file *f)
+{
+ return &f->f_path;
+}
+
static inline struct file *file_clone_open(struct file *file)
{
return dentry_open(&file->f_path, file->f_flags, file->f_cred);
diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
index db575094c498..d8b302d01083 100644
--- a/kernel/trace/trace_output.c
+++ b/kernel/trace/trace_output.c
@@ -404,7 +404,7 @@ static int seq_print_user_ip(struct trace_seq *s, struct mm_struct *mm,
vmstart = vma->vm_start;
}
if (file) {
- ret = trace_seq_path(s, &file->f_path);
+ ret = trace_seq_path(s, file_user_path(file));
if (ret)
trace_seq_printf(s, "[+0x%lx]",
ip - vmstart);
--
2.34.1