procfs: implement /proc/self/exe

This patch adds /proc/self/exe which symlinks
to the application being executed. It also adds symlink
support to procfs.

Signed-off-by: Waldemar Kozaczuk <[email protected]>

fs/procfs/procfs_vnops.cc

@@ -7,6 +7,7 @@
 
 #include <unistd.h>
 
+#include <osv/app.hh>
 #include <osv/mount.h>
 #include <osv/prex.h>
 #include <osv/sched.hh>
@@ -144,6 +145,17 @@ static std::string procfs_hostname()
     return std::string(hostname);
 }
 
+static std::string procfs_exe()
+{
+    auto app = sched::thread::current_app();
+
+    if (app && app->lib()) {
+        return app->lib()->pathname();
+    } else {
+        return "";
+    }
+}
+
 static int
 procfs_mount(mount* mp, const char *dev, int flags, const void* data)
 {
@@ -154,6 +166,9 @@ procfs_mount(mount* mp, const char *dev, int flags, const void* data)
     self->add("stat", inode_count++, procfs_stats);
     self->add("status", inode_count++, procfs_status);
 
+    auto exe = make_shared<pseudo_symlink_node>(inode_count++, procfs_exe);
+    self->add("exe", exe);
+
     auto kernel = make_shared<pseudo_dir_node>(inode_count++);
     kernel->add("hostname", inode_count++, procfs_hostname);
 
@@ -216,7 +231,7 @@ vnops procfs_vnops = {
     (vnop_link_t)     vop_eperm,  // vop_link
     (vnop_cache_t)     nullptr,   // vop_arc
     (vnop_fallocate_t) vop_nullop, // vop_fallocate
-    (vnop_readlink_t)  vop_nullop, // vop_readlink
+    pseudofs::readlink,            // vop_readlink
     (vnop_symlink_t)   vop_nullop, // vop_symlink
 };
 

fs/pseudofs/pseudofs.cc

@@ -27,6 +27,12 @@ static pseudo_file_node *to_file_node(vnode *vp) {
     return dynamic_cast<pseudo_file_node *>(np);
 }
 
+static pseudo_symlink_node *to_symlink_node(vnode *vp) {
+    auto *np = to_node(vp);
+
+    return dynamic_cast<pseudo_symlink_node *>(np);
+}
+
 int open(file *fp) {
     auto *np = to_file_node(fp->f_dentry->d_vnode);
     if (np) {
@@ -65,6 +71,18 @@ int read(vnode *vp, file *fp, uio *uio, int ioflags) {
     return uiomove(const_cast<char *>(data->data()) + uio->uio_offset, len, uio);
 }
 
+int readlink(vnode *vp, uio *uio) {
+    if (vp->v_type != VLNK)
+        return EINVAL;
+
+    auto *np = to_symlink_node(vp);
+    auto *target_path = np->target_path();
+    if (uio->uio_offset >= (off_t) target_path->size())
+        return 0;
+
+    return uiomove(const_cast<char *>(target_path->data()) + uio->uio_offset, target_path->size(), uio);
+}
+
 int write(vnode *vp, uio *uio, int ioflags) {
     return EINVAL;
 }
@@ -135,6 +153,8 @@ int readdir(vnode *vp, file *fp, dirent *dir) {
         auto np = dir_entry->second;
         if (np->type() == VDIR) {
             dir->d_type = DT_DIR;
+        } else if (np->type() == VLNK) {
+            dir->d_type = DT_LNK;
         } else {
             dir->d_type = DT_REG;
         }

fs/pseudofs/pseudofs.hh

@@ -62,6 +62,27 @@ private:
     function<string()> _gen;
 };
 
+class pseudo_symlink_node : public pseudo_node {
+public:
+    pseudo_symlink_node(uint64_t ino, function<string()> gen)
+            : pseudo_node(ino, VLNK), _target_path_gen(gen) {}
+
+    virtual off_t size() const override {
+        return 0;
+    }
+
+    virtual mode_t mode() const override {
+        return S_IRUSR | S_IRGRP | S_IROTH;
+    }
+
+    string *target_path() const {
+        return new string(_target_path_gen());
+    }
+
+private:
+    function<string()> _target_path_gen;
+};
+
 class pseudo_dir_node : public pseudo_node {
 public:
     pseudo_dir_node(uint64_t ino) : pseudo_node(ino, VDIR) {}
@@ -112,6 +133,8 @@ int close(vnode *vp, file *fp);
 
 int read(vnode *vp, file *fp, uio *uio, int ioflags);
 
+int readlink(vnode *vp, uio *uio);
+
 int write(vnode *vp, uio *uio, int ioflags);
 
 int ioctl(vnode *vp, file *fp, u_long cmd, void *arg);