Re: [PATCH v2] stdlib: realpath use malloc replace __alloca to reduce stack overflow risks [BZ #26341]

[Paul Eggert <eggert@cs.ucla.edu>]

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On 8/7/20 3:16 AM, Xiaoming Ni wrote:
> Realpath() cyclically invokes __alloca() when processing soft link files,
> which may consume 164 KB stack space.
> Therefore, replace __alloca with malloc to reduce stack overflow risks

I don't understand why malloc is required here.

Isn't the problem that the buffer is being allocated over and over again? And if 
you fix that bug, the function should allocate only 8 KiB via __alloca on 
GNU/Linux. Something like the attached (untested) patch, say. So why not keep 
using the stack? That would be more efficient than resorting to the heap.

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diff --git a/stdlib/canonicalize.c b/stdlib/canonicalize.c
index cbd885a3c5..ddeea3e2e9 100644
--- a/stdlib/canonicalize.c
+++ b/stdlib/canonicalize.c
@@ -42,7 +42,7 @@
 char *
 __realpath (const char *name, char *resolved)
 {
-  char *rpath, *dest, *extra_buf = NULL;
+  char *rpath, *dest, *buf = NULL, *extra_buf = NULL;
   const char *start, *end, *rpath_limit;
   long int path_max;
   int num_links = 0;
@@ -163,7 +163,6 @@ __realpath (const char *name, char *resolved)
 
 	  if (S_ISLNK (st.st_mode))
 	    {
-	      char *buf = __alloca (path_max);
 	      size_t len;
 
 	      if (++num_links > __eloop_threshold ())
@@ -172,6 +171,8 @@ __realpath (const char *name, char *resolved)
 		  goto error;
 		}
 
+	      if (!buf)
+		buf = __alloca (path_max);
 	      n = __readlink (rpath, buf, path_max - 1);
 	      if (n < 0)
 		goto error;