proc: less code duplication in /proc/*/cmdline

After staring at this code for a while I've figured using small 2-entry
array describing ARGV and ENVP is the way to address code duplication
critique.

Link: http://lkml.kernel.org/r/20170105185724.GA12027@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -291,102 +291,70 @@ static ssize_t proc_pid_cmdline_read(struct file *file, char __user *buf,
 			rv	+= nr_read;
 		}
 	} else {
-		/*
-		 * Command line (1 string) occupies ARGV and maybe
-		 * extends into ENVP.
-		 */
-		if (len1 + len2 <= *pos)
-			goto skip_argv_envp;
-		if (len1 <= *pos)
-			goto skip_argv;
-
-		p = arg_start + *pos;
-		len = len1 - *pos;
-		while (count > 0 && len > 0) {
-			unsigned int _count, l;
-			int nr_read;
-			bool final;
-
-			_count = min3(count, len, PAGE_SIZE);
-			nr_read = access_remote_vm(mm, p, page, _count, 0);
-			if (nr_read < 0)
-				rv = nr_read;
-			if (nr_read <= 0)
-				goto out_free_page;
-
-			/*
-			 * Command line can be shorter than whole ARGV
-			 * even if last "marker" byte says it is not.
-			 */
-			final = false;
-			l = strnlen(page, nr_read);
-			if (l < nr_read) {
-				nr_read = l;
-				final = true;
-			}
-
-			if (copy_to_user(buf, page, nr_read)) {
-				rv = -EFAULT;
-				goto out_free_page;
-			}
-
-			p	+= nr_read;
-			len	-= nr_read;
-			buf	+= nr_read;
-			count	-= nr_read;
-			rv	+= nr_read;
-
-			if (final)
-				goto out_free_page;
-		}
-skip_argv:
 		/*
 		 * Command line (1 string) occupies ARGV and
 		 * extends into ENVP.
 		 */
-		if (len1 <= *pos) {
-			p = env_start + *pos - len1;
-			len = len1 + len2 - *pos;
-		} else {
-			p = env_start;
-			len = len2;
+		struct {
+			unsigned long p;
+			unsigned long len;
+		} cmdline[2] = {
+			{ .p = arg_start, .len = len1 },
+			{ .p = env_start, .len = len2 },
+		};
+		loff_t pos1 = *pos;
+		unsigned int i;
+
+		i = 0;
+		while (i < 2 && pos1 >= cmdline[i].len) {
+			pos1 -= cmdline[i].len;
+			i++;
 		}
-		while (count > 0 && len > 0) {
-			unsigned int _count, l;
-			int nr_read;
-			bool final;
-
-			_count = min3(count, len, PAGE_SIZE);
-			nr_read = access_remote_vm(mm, p, page, _count, 0);
-			if (nr_read < 0)
-				rv = nr_read;
-			if (nr_read <= 0)
-				goto out_free_page;
-
-			/* Find EOS. */
-			final = false;
-			l = strnlen(page, nr_read);
-			if (l < nr_read) {
-				nr_read = l;
-				final = true;
-			}
-
-			if (copy_to_user(buf, page, nr_read)) {
-				rv = -EFAULT;
-				goto out_free_page;
+		while (i < 2) {
+			p = cmdline[i].p + pos1;
+			len = cmdline[i].len - pos1;
+			while (count > 0 && len > 0) {
+				unsigned int _count, l;
+				int nr_read;
+				bool final;
+
+				_count = min3(count, len, PAGE_SIZE);
+				nr_read = access_remote_vm(mm, p, page, _count, 0);
+				if (nr_read < 0)
+					rv = nr_read;
+				if (nr_read <= 0)
+					goto out_free_page;
+
+				/*
+				 * Command line can be shorter than whole ARGV
+				 * even if last "marker" byte says it is not.
+				 */
+				final = false;
+				l = strnlen(page, nr_read);
+				if (l < nr_read) {
+					nr_read = l;
+					final = true;
+				}
+
+				if (copy_to_user(buf, page, nr_read)) {
+					rv = -EFAULT;
+					goto out_free_page;
+				}
+
+				p	+= nr_read;
+				len	-= nr_read;
+				buf	+= nr_read;
+				count	-= nr_read;
+				rv	+= nr_read;
+
+				if (final)
+					goto out_free_page;
 			}
 
-			p	+= nr_read;
-			len	-= nr_read;
-			buf	+= nr_read;
-			count	-= nr_read;
-			rv	+= nr_read;
-
-			if (final)
-				goto out_free_page;
+			/* Only first chunk can be read partially. */
+			pos1 = 0;
+			i++;
 		}
-skip_argv_envp:
-		;
 	}
 
 out_free_page: