[glibc] hurd: writev: Get rid of alloca

[glibc] hurd: writev: Get rid of alloca

[Samuel Thibault]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=cf30aa43a5917f441c9438aaee201c53c8e1d76b

commit cf30aa43a5917f441c9438aaee201c53c8e1d76b
Author: Joe Simmons-Talbott <josimmon@redhat.com>
Date:   Thu Jun 8 11:58:43 2023 -0400

    hurd: writev: Get rid of alloca
    
    Use a scratch_buffer rather than alloca to avoid potential stack
    overflows.
    
    Checked on i686-gnu and x86_64-linux-gnu
    Message-Id: <20230608155844.976554-1-josimmon@redhat.com>

Diff:
---
 sysdeps/posix/writev.c | 37 ++++++++++++++-----------------------
 1 file changed, 14 insertions(+), 23 deletions(-)

diff --git a/sysdeps/posix/writev.c b/sysdeps/posix/writev.c
index 53e090c087..d4c3cf6f03 100644
--- a/sysdeps/posix/writev.c
+++ b/sysdeps/posix/writev.c
@@ -19,19 +19,13 @@
 #include <unistd.h>
 #include <string.h>
 #include <limits.h>
+#include <scratch_buffer.h>
 #include <stdbool.h>
 #include <sys/param.h>
 #include <sys/uio.h>
 #include <errno.h>
 
 
-static void
-ifree (char **ptrp)
-{
-  free (*ptrp);
-}
-
-
 /* Write data pointed by the buffers described by VECTOR, which
    is a vector of COUNT 'struct iovec's, to file descriptor FD.
    The data is written in the order specified.
@@ -53,22 +47,17 @@ __writev (int fd, const struct iovec *vector, int count)
       bytes += vector[i].iov_len;
     }
 
-  /* Allocate a temporary buffer to hold the data.  We should normally
-     use alloca since it's faster and does not require synchronization
-     with other threads.  But we cannot if the amount of memory
-     required is too large.  */
-  char *buffer;
-  char *malloced_buffer __attribute__ ((__cleanup__ (ifree))) = NULL;
-  if (__libc_use_alloca (bytes))
-    buffer = (char *) __alloca (bytes);
-  else
-    {
-      malloced_buffer = buffer = (char *) malloc (bytes);
-      if (buffer == NULL)
-	/* XXX I don't know whether it is acceptable to try writing
-	   the data in chunks.  Probably not so we just fail here.  */
-	return -1;
-    }
+  /* Allocate a temporary buffer to hold the data.  Use a scratch_buffer
+     since it's faster for small buffer sizes but can handle larger
+     allocations as well.  */
+
+  struct scratch_buffer buf;
+  scratch_buffer_init (&buf);
+  if (!scratch_buffer_set_array_size (&buf, 1, bytes))
+    /* XXX I don't know whether it is acceptable to try writing
+       the data in chunks.  Probably not so we just fail here.  */
+    return -1;
+  char *buffer = buf.data;
 
   /* Copy the data into BUFFER.  */
   size_t to_copy = bytes;
@@ -86,6 +75,8 @@ __writev (int fd, const struct iovec *vector, int count)
 
   ssize_t bytes_written = __write (fd, buffer, bytes);
 
+  scratch_buffer_free (&buf);
+
   return bytes_written;
 }
 libc_hidden_def (__writev)