Fix race condition in pg_ctl reading postmaster.pid.

hlinnaka · hlinnaka · commit 1c95b5eea6be · 2012-10-15T10:54:33.000+03:00
If postmaster changed postmaster.pid while pg_ctl was reading it, pg_ctl
could overrun the buffer it allocated for the file. Fix by reading the
whole file to memory with one read() call.

initdb contains an identical copy of the readfile() function, but the files
that initdb reads are static, not modified concurrently. Nevertheless, add
a simple bounds-check there, if only to silence static analysis tools.

Per report from Dave Vitek. Backpatch to all supported branches.
diff --git a/src/bin/initdb/initdb.c b/src/bin/initdb/initdb.c
@@ -368,6 +368,7 @@ readfile(const char *path)
 	int			maxlength = 1,
 				linelen = 0;
 	int			nlines = 0;
+	int			n;
 	char	  **result;
 	char	   *buffer;
 	int			c;
@@ -405,13 +406,13 @@ readfile(const char *path)
 
 	/* now reprocess the file and store the lines */
 	rewind(infile);
-	nlines = 0;
-	while (fgets(buffer, maxlength + 1, infile) != NULL)
-		result[nlines++] = xstrdup(buffer);
+	n = 0;
+	while (fgets(buffer, maxlength + 1, infile) != NULL && n < nlines)
+		result[n++] = xstrdup(buffer);
 
 	fclose(infile);
 	free(buffer);
-	result[nlines] = NULL;
+	result[n] = NULL;
 
 	return result;
 }
diff --git a/src/bin/pg_ctl/pg_ctl.c b/src/bin/pg_ctl/pg_ctl.c
@@ -20,6 +20,7 @@
 #include "postgres_fe.h"
 #include "libpq-fe.h"
 
+#include <fcntl.h>
 #include <locale.h>
 #include <signal.h>
 #include <sys/types.h>
@@ -297,50 +298,77 @@ get_pgpid(void)
 static char **
 readfile(const char *path)
 {
-	FILE	   *infile;
-	int			maxlength = 1,
-				linelen = 0;
-	int			nlines = 0;
+	int			fd;
+	int			nlines;
 	char	  **result;
 	char	   *buffer;
-	int			c;
+	char	   *linebegin;
+	int			i;
+	int			n;
+	int			len;
+	struct stat	statbuf;
 
-	if ((infile = fopen(path, "r")) == NULL)
+	/*
+	 * Slurp the file into memory.
+	 *
+	 * The file can change concurrently, so we read the whole file into memory
+	 * with a single read() call. That's not guaranteed to get an atomic
+	 * snapshot, but in practice, for a small file, it's close enough for the
+	 * current use.
+	 */
+	fd = open(path, O_RDONLY | PG_BINARY, 0);
+	if (fd < 0)
 		return NULL;
+	if (fstat(fd, &statbuf) < 0)
+		return NULL;
+	if (statbuf.st_size == 0)
+	{
+		/* empty file */
+		result = (char **) pg_malloc(sizeof(char *));
+		*result = NULL;
+		return result;
+	}
+	buffer = pg_malloc(statbuf.st_size + 1);
 
-	/* pass over the file twice - the first time to size the result */
+	len = read(fd, buffer, statbuf.st_size + 1);
+	close(fd);
+	if (len != statbuf.st_size)
+	{
+		/* oops, the file size changed between fstat and read */
+		free(buffer);
+		return NULL;
+	}
 
-	while ((c = fgetc(infile)) != EOF)
+	/* count newlines */
+	nlines = 0;
+	for (i = 0; i < len - 1; i++)
 	{
-		linelen++;
-		if (c == '\n')
-		{
+		if (buffer[i] == '\n')
 			nlines++;
-			if (linelen > maxlength)
-				maxlength = linelen;
-			linelen = 0;
-		}
 	}
+	nlines++; /* account for the last line */
 
-	/* handle last line without a terminating newline (yuck) */
-	if (linelen)
-		nlines++;
-	if (linelen > maxlength)
-		maxlength = linelen;
-
-	/* set up the result and the line buffer */
+	/* set up the result buffer */
 	result = (char **) pg_malloc((nlines + 1) * sizeof(char *));
-	buffer = (char *) pg_malloc(maxlength + 1);
 
-	/* now reprocess the file and store the lines */
-	rewind(infile);
-	nlines = 0;
-	while (fgets(buffer, maxlength + 1, infile) != NULL)
-		result[nlines++] = xstrdup(buffer);
+	/* now split the buffer into lines */
+	linebegin = buffer;
+	n = 0;
+	for (i = 0; i < len; i++)
+	{
+		if (buffer[i] == '\n' || i == len - 1)
+		{
+			int		slen = &buffer[i] - linebegin + 1;
+			char   *linebuf = pg_malloc(slen + 1);
+			memcpy(linebuf, linebegin, slen);
+			linebuf[slen] = '\0';
+			result[n++] = linebuf;
+			linebegin = &buffer[i + 1];
+		}
+	}
+	result[n] = NULL;
 
-	fclose(infile);
 	free(buffer);
-	result[nlines] = NULL;
 
 	return result;
 }
