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libbpf-tools/opensnoop: support mount full-path #5333

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Merged
yonghong-song merged 1 commit into from
Jun 15, 2025
Merged

libbpf-tools/opensnoop: support mount full-path #5333

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36 changes: 30 additions & 6 deletions libbpf-tools/opensnoop.bpf.c
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Original file line number Diff line number Diff line change
Expand Up @@ -155,13 +155,18 @@ int trace_exit(struct syscall_trace_exit* ctx)
if (full_path && eventp->fname[0] != '/') {
int depth;
struct task_struct *task;
struct dentry *dentry, *parent_dentry;
struct dentry *dentry, *parent_dentry, *mnt_root;
struct vfsmount *vfsmnt;
struct mount *mnt;
size_t filepart_length;
void *payload = eventp->fname;
char *payload = eventp->fname;


task = (struct task_struct *)bpf_get_current_task_btf();
dentry = BPF_CORE_READ(task, fs, pwd.dentry);
vfsmnt = BPF_CORE_READ(task, fs, pwd.mnt);
mnt = container_of(vfsmnt, struct mount, mnt);
mnt_root = BPF_CORE_READ(vfsmnt, mnt_root);

for (depth = 1, payload += NAME_MAX; depth < MAX_PATH_DEPTH; depth++) {
filepart_length =
Expand All @@ -173,13 +178,32 @@ int trace_exit(struct syscall_trace_exit* ctx)
break;
}

parent_dentry = BPF_CORE_READ(dentry, d_parent);
if (dentry == parent_dentry)
break;

if (filepart_length > NAME_MAX)
break;

parent_dentry = BPF_CORE_READ(dentry, d_parent);

if (dentry == parent_dentry || dentry == mnt_root) {
struct mount *mnt_parent;
mnt_parent = BPF_CORE_READ(mnt, mnt_parent);

if (mnt != mnt_parent) {
dentry = BPF_CORE_READ(mnt, mnt_mountpoint);
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@yonghong-song yonghong-song Jun 13, 2025

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IIUC, correctly, dentry here is a 'hidden' dentry to facilitate path traversal. So after this, we should do dentry = BPF_CORE_READ(dentry, d_parent);?

Please look at this upstream patch https://lore.kernel.org/bpf/20250611220220.3681382-2-song@kernel.org/ to double check.

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@Rtoax Rtoax Jun 14, 2025 •
edited
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I tried, but tracking the wrong path

diff --git a/libbpf-tools/opensnoop.bpf.c b/libbpf-tools/opensnoop.bpf.c
index 64d931a8..08510e42 100644
--- a/libbpf-tools/opensnoop.bpf.c
+++ b/libbpf-tools/opensnoop.bpf.c
@@ -189,6 +189,7 @@ int trace_exit(struct syscall_trace_exit* ctx)
 
                                if (mnt != mnt_parent) {
                                        dentry = BPF_CORE_READ(mnt, mnt_mountpoint);
+                                       dentry = BPF_CORE_READ(dentry, d_parent);
 
                                        mnt = mnt_parent;
                                        vfsmnt = &mnt->mnt;

For example:

    $ mount | grep sda
    /dev/sda on /home/sda type xfs ...

    /home/sda/rongtao/a.out
          ^^^
          [1]

    /* get 'rongtao' mount point 'sda' */
[1] dentry = BPF_CORE_READ(mnt, mnt_mountpoint);
[2] dentry = BPF_CORE_READ(dentry, d_parent);

so, i think there is no need [2] after [1], right?

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@yonghong-song yonghong-song Jun 15, 2025

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I tried a nested mount example. You are right, the [2] is not needed.


mnt = mnt_parent;
vfsmnt = &mnt->mnt;

mnt_root = BPF_CORE_READ(vfsmnt, mnt_root);

eventp->path_depth++;
payload += NAME_MAX;
continue;
} else {
/* Real root directory */
break;
}
}

payload += NAME_MAX;

dentry = parent_dentry;
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14 changes: 12 additions & 2 deletions libbpf-tools/opensnoop.c
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Original file line number Diff line number Diff line change
Expand Up @@ -268,7 +268,16 @@ int handle_event(void *ctx, void *data, size_t data_sz)
sps_cnt += 9;
}
if (env.full_path) {
for (int depth = e.path_depth; depth >= 0; depth--)
for (int depth = e.path_depth; depth >= 0; depth--) {
char *fname = (char *)&e.fname[NAME_MAX * depth];

/**
* If it is a mount point, there will be a '/', because
* the '/' will be added below, so just skip this '/'.
*/
if (fname[0] == '/' && fname[1] == '\0')
continue;

/**
* 1. If the file/path name starts with '/', do not
* print the '/' prefix.
Expand All @@ -281,7 +290,8 @@ int handle_event(void *ctx, void *data, size_t data_sz)
"/\0" + (e.fname[NAME_MAX * depth] == '/' ||
((e.get_path_failed || e.path_depth == MAX_PATH_DEPTH - 1) &&
depth == e.path_depth)),
(char *)&e.fname[NAME_MAX * depth]);
fname);
}
printf("\n");
} else
printf("%s\n", e.fname);
Expand Down
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