[v4,2/2] glob: implement GLOB_ALTDIRFUNC et al

Submitted by Ismael Luceno on Sept. 6, 2019, 7:40 p.m.

Details

Message ID 20190906194022.23577-2-ismael@iodev.co.uk
State New
Series "Series without cover letter"
Headers show

Commit Message

Ismael Luceno Sept. 6, 2019, 7:40 p.m.
Signed-off-by: Ismael Luceno <ismael@iodev.co.uk>
---

Notes:
    Changes since v3:
    
    - Wrap pointers used by GLOB_ALTDIRFUNC on the header to protect from namespace
      contamination. GNU and BSD code shouldn't be using "dirent", so it should be
      safe.
    
    Changes since v2:
    
    - Rebased
    
    Changes since v1:
    
    - Avoid overwriting the function pointers in glob_t
    - Wrap {open,read}dir too

 include/glob.h   | 10 ++++++++++
 src/regex/glob.c | 47 +++++++++++++++++++++++++++++++++++++----------
 2 files changed, 47 insertions(+), 10 deletions(-)

Patch hide | download patch | download mbox

diff --git a/include/glob.h b/include/glob.h
index 0ff70bdfeef2..26ea65cd1097 100644
--- a/include/glob.h
+++ b/include/glob.h
@@ -16,7 +16,16 @@  typedef struct {
 	char **gl_pathv;
 	size_t gl_offs;
 	int __dummy1;
+
+#if defined(_BSD_SOURCE) || defined(_GNU_SOURCE)
+	void (*gl_closedir)(void *);
+	struct dirent *(*gl_readdir)(void *);
+	void *(*gl_opendir)(const char *);
+	int (*gl_lstat)(const char *__restrict, struct stat *__restrict);
+	int (*gl_stat)(const char *__restrict, struct stat *__restrict);
+#else
 	void *__dummy2[5];
+#endif
 } glob_t;
 
 int  glob(const char *__restrict, int, int (*)(const char *, int), glob_t *__restrict);
@@ -31,6 +40,7 @@  void globfree(glob_t *);
 #define GLOB_NOESCAPE 0x40
 #define	GLOB_PERIOD   0x80
 
+#define GLOB_ALTDIRFUNC  0x0200
 #define GLOB_NOMAGIC     0x0800
 #define GLOB_TILDE       0x1000
 #define GLOB_TILDE_CHECK 0x4000
diff --git a/src/regex/glob.c b/src/regex/glob.c
index 7780e21ee113..4f329f053fe0 100644
--- a/src/regex/glob.c
+++ b/src/regex/glob.c
@@ -1,7 +1,7 @@ 
 #define _BSD_SOURCE
+#include <sys/stat.h>
 #include <glob.h>
 #include <fnmatch.h>
-#include <sys/stat.h>
 #include <dirent.h>
 #include <limits.h>
 #include <string.h>
@@ -11,6 +11,14 @@ 
 #include <unistd.h>
 #include <pwd.h>
 
+struct dirfn {
+	void (*closedir)(void *);
+	struct dirent *(*readdir)(void *);
+	void *(*opendir)(const char *);
+	int (*lstat)(const char *restrict, struct stat *restrict);
+	int (*stat)(const char *restrict, struct stat *restrict);
+};
+
 struct match
 {
 	struct match *next;
@@ -32,7 +40,7 @@  static int append(struct match **tail, const char *name, size_t len, int mark)
 	return 0;
 }
 
-static int do_glob(char *buf, size_t pos, int type, char *pat, int flags, int (*errfunc)(const char *path, int err), struct match **tail)
+static int do_glob(char *buf, size_t pos, int type, char *pat, int flags, int (*errfunc)(const char *path, int err), const struct dirfn * const restrict dirfn, struct match **tail)
 {
 	/* If GLOB_MARK is unused, we don't care about type. */
 	if (!type && !(flags & GLOB_MARK)) type = DT_REG;
@@ -100,11 +108,11 @@  static int do_glob(char *buf, size_t pos, int type, char *pat, int flags, int (*
 		 * or if that fails, use lstat for determining existence to
 		 * avoid false negatives in the case of broken symlinks. */
 		struct stat st;
-		if ((flags & GLOB_MARK) && (!type||type==DT_LNK) && !stat(buf, &st)) {
+		if ((flags & GLOB_MARK) && (!type||type==DT_LNK) && !dirfn->stat(buf, &st)) {
 			if (S_ISDIR(st.st_mode)) type = DT_DIR;
 			else type = DT_REG;
 		}
-		if (!type && lstat(buf, &st)) {
+		if (!type && dirfn->lstat(buf, &st)) {
 			if (errno!=ENOENT && (errfunc(buf, errno) || (flags & GLOB_ERR)))
 				return GLOB_ABORTED;
 			return 0;
@@ -124,7 +132,7 @@  static int do_glob(char *buf, size_t pos, int type, char *pat, int flags, int (*
 			saved_sep = '\\';
 		}
 	}
-	DIR *dir = opendir(pos ? buf : ".");
+	DIR *dir = dirfn->opendir(pos ? buf : ".");
 	if (!dir) {
 		if (errfunc(buf, errno) || (flags & GLOB_ERR))
 			return GLOB_ABORTED;
@@ -132,7 +140,7 @@  static int do_glob(char *buf, size_t pos, int type, char *pat, int flags, int (*
 	}
 	int old_errno = errno;
 	struct dirent *de;
-	while (errno=0, de=readdir(dir)) {
+	while (errno=0, de=dirfn->readdir(dir)) {
 		/* Quickly skip non-directories when there's pattern left. */
 		if (p2 && de->d_type && de->d_type!=DT_DIR && de->d_type!=DT_LNK)
 			continue;
@@ -157,15 +165,15 @@  static int do_glob(char *buf, size_t pos, int type, char *pat, int flags, int (*
 
 		memcpy(buf+pos, de->d_name, l+1);
 		if (p2) *p2 = saved_sep;
-		int r = do_glob(buf, pos+l, de->d_type, p2 ? p2 : "", flags, errfunc, tail);
+		int r = do_glob(buf, pos+l, de->d_type, p2 ? p2 : "", flags, errfunc, dirfn, tail);
 		if (r) {
-			closedir(dir);
+			dirfn->closedir(dir);
 			return r;
 		}
 	}
 	int readerr = errno;
 	if (p2) *p2 = saved_sep;
-	closedir(dir);
+	dirfn->closedir(dir);
 	if (readerr && (errfunc(buf, errno) || (flags & GLOB_ERR)))
 		return GLOB_ABORTED;
 	errno = old_errno;
@@ -224,6 +232,10 @@  static int expand_tilde(char **pat, char *buf, size_t *pos)
 	return 0;
 }
 
+static void wrap_closedir(void *p) { __closedir(p); }
+static struct dirent *wrap_readdir(void *d) { return __readdir(d); }
+static void *wrap_opendir(const char *path) { return __opendir(path); }
+
 int glob(const char *restrict pat, int flags, int (*errfunc)(const char *path, int err), glob_t *restrict g)
 {
 	struct match head = { .next = NULL }, *tail = &head;
@@ -231,9 +243,24 @@  int glob(const char *restrict pat, int flags, int (*errfunc)(const char *path, i
 	size_t offs = (flags & GLOB_DOOFFS) ? g->gl_offs : 0;
 	int error = 0;
 	char buf[PATH_MAX];
+	struct dirfn dirfn;
 	
 	if (!errfunc) errfunc = ignore_err;
 
+	if (flags & GLOB_ALTDIRFUNC) {
+		dirfn.closedir = g->gl_closedir;
+		dirfn.readdir = g->gl_readdir;
+		dirfn.opendir = g->gl_opendir;
+		dirfn.lstat = g->gl_lstat;
+		dirfn.stat = g->gl_stat;
+	} else {
+		dirfn.closedir = wrap_closedir;
+		dirfn.readdir = wrap_readdir;
+		dirfn.opendir = wrap_opendir;
+		dirfn.lstat = lstat;
+		dirfn.stat = stat;
+	}
+
 	if (!(flags & GLOB_APPEND)) {
 		g->gl_offs = offs;
 		g->gl_pathc = 0;
@@ -249,7 +276,7 @@  int glob(const char *restrict pat, int flags, int (*errfunc)(const char *path, i
 		if ((flags & (GLOB_TILDE | GLOB_TILDE_CHECK)) && *p == '~')
 			error = expand_tilde(&s, buf, &pos);
 		if (!error)
-			error = do_glob(buf, pos, 0, s, flags, errfunc, &tail);
+			error = do_glob(buf, pos, 0, s, flags, errfunc, &dirfn, &tail);
 		free(p);
 	}
 

Comments

Rich Felker Sept. 6, 2019, 9:42 p.m.
On Fri, Sep 06, 2019 at 09:40:22PM +0200, Ismael Luceno wrote:
> diff --git a/include/glob.h b/include/glob.h
> index 0ff70bdfeef2..26ea65cd1097 100644
> --- a/include/glob.h
> +++ b/include/glob.h
> @@ -16,7 +16,16 @@ typedef struct {
>  	char **gl_pathv;
>  	size_t gl_offs;
>  	int __dummy1;
> +
> +#if defined(_BSD_SOURCE) || defined(_GNU_SOURCE)
> +	void (*gl_closedir)(void *);
> +	struct dirent *(*gl_readdir)(void *);
> +	void *(*gl_opendir)(const char *);
> +	int (*gl_lstat)(const char *__restrict, struct stat *__restrict);
> +	int (*gl_stat)(const char *__restrict, struct stat *__restrict);
> +#else
>  	void *__dummy2[5];
> +#endif
>  } glob_t;

I think you need to forward-declare at least struct stat, and possibly
also struct dirent, so that they refer to the same types as the
declarations elsewhere. At least the argument list is a different
scope and declares a *different* struct stat shadowing the file-scope
one if the file-scope one is not already declared.

With the above change and:

> diff --git a/src/regex/glob.c b/src/regex/glob.c
> index 7780e21ee113..4f329f053fe0 100644
> --- a/src/regex/glob.c
> +++ b/src/regex/glob.c
> @@ -1,7 +1,7 @@
>  #define _BSD_SOURCE
           ^^^^^^^^^^^

combined, every use of g->[something] in glob() is now undefined
behavior if the caller was using a standard profile where the
__dummy2-containing version of glob_t above is the one that's used.
That's because it's accessing one struct type via an lvalue of a
different struct type.

> +#include <sys/stat.h>
>  #include <glob.h>
>  #include <fnmatch.h>
> -#include <sys/stat.h>

Unrelated: this change is gratuitous.

> @@ -224,6 +232,10 @@ static int expand_tilde(char **pat, char *buf, size_t *pos)
>  	return 0;
>  }
>  
> +static void wrap_closedir(void *p) { __closedir(p); }
> +static struct dirent *wrap_readdir(void *d) { return __readdir(d); }
> +static void *wrap_opendir(const char *path) { return __opendir(path); }
> +
>  int glob(const char *restrict pat, int flags, int (*errfunc)(const char *path, int err), glob_t *restrict g)
>  {
>  	struct match head = { .next = NULL }, *tail = &head;
> @@ -231,9 +243,24 @@ int glob(const char *restrict pat, int flags, int (*errfunc)(const char *path, i
>  	size_t offs = (flags & GLOB_DOOFFS) ? g->gl_offs : 0;
                                              ^^^^^^^^^^

These are the accesses that become UB. To make them well-defined,
they'd need to be something like G(g,gl_offs), where

#define G(g,m) (*(1?(void*)((char*)(g)+offsetof(glob_t,m)):&((glob_t){0}).m)

is the cleanest abstraction I can think of for the monstrosity needed
to access "something layout-compatible with glob_t, but not glob_t, at
address g".

Imposing this ugliness to avoid UB is part of why I'm hesitant about
this functionality. I think there should be good motivation for it if
we want to go forward with it.

>  	int error = 0;
>  	char buf[PATH_MAX];
> +	struct dirfn dirfn;
>  	
>  	if (!errfunc) errfunc = ignore_err;
>  
> +	if (flags & GLOB_ALTDIRFUNC) {
> +		dirfn.closedir = g->gl_closedir;
> +		dirfn.readdir = g->gl_readdir;
> +		dirfn.opendir = g->gl_opendir;
> +		dirfn.lstat = g->gl_lstat;
> +		dirfn.stat = g->gl_stat;
> +	} else {
> +		dirfn.closedir = wrap_closedir;
> +		dirfn.readdir = wrap_readdir;
> +		dirfn.opendir = wrap_opendir;
> +		dirfn.lstat = lstat;
> +		dirfn.stat = stat;
> +	}
> +
>  	if (!(flags & GLOB_APPEND)) {
>  		g->gl_offs = offs;
>  		g->gl_pathc = 0;
> @@ -249,7 +276,7 @@ int glob(const char *restrict pat, int flags, int (*errfunc)(const char *path, i
>  		if ((flags & (GLOB_TILDE | GLOB_TILDE_CHECK)) && *p == '~')
>  			error = expand_tilde(&s, buf, &pos);
>  		if (!error)
> -			error = do_glob(buf, pos, 0, s, flags, errfunc, &tail);
> +			error = do_glob(buf, pos, 0, s, flags, errfunc, &dirfn, &tail);
>  		free(p);
>  	}

Not necessary for correctness, but if we're adding a context structure
that gets passed down to each level of do_glob, we might as well put
the errfunc in it too, and possibly also flags and &tail in it, so
that we reduce the amount of state at each recursion frame rather than
increasing it.

It's likely this would make more sense to do as a separate patch
though, to keep the diffs clear.

Rich