From e1e52afa08eadcc61871cce000ac8c9db1a7c24d Mon Sep 17 00:00:00 2001
From: Alexander Kabaev <kan@FreeBSD.org>
Date: Sun, 19 Jun 2011 13:35:41 +0000
Subject: [PATCH] Minimize backward seeks when trying to load ELF relocatable
 modules.

Some of loader filesystems are very ill equipped to handle seeking
backwards within the file. Namely, tftp requires trasfer to be
restarted from the start of the file every time we go backwards.
---
 sys/boot/common/load_elf_obj.c | 37 +++++++++++++++++++++++++++-------
 1 file changed, 30 insertions(+), 7 deletions(-)

diff --git a/sys/boot/common/load_elf_obj.c b/sys/boot/common/load_elf_obj.c
index dcd71efff38..54d9b103f64 100644
--- a/sys/boot/common/load_elf_obj.c
+++ b/sys/boot/common/load_elf_obj.c
@@ -196,7 +196,7 @@ static int
 __elfN(obj_loadimage)(struct preloaded_file *fp, elf_file_t ef, u_int64_t off)
 {
 	Elf_Ehdr *hdr;
-	Elf_Shdr *shdr;
+	Elf_Shdr *shdr, *cshdr, *lshdr;
 	vm_offset_t firstaddr, lastaddr;
 	int i, nsym, res, ret, shdrbytes, symstrindex;
 
@@ -294,12 +294,35 @@ __elfN(obj_loadimage)(struct preloaded_file *fp, elf_file_t ef, u_int64_t off)
 	/* Clear the whole area, including bss regions. */
 	kern_bzero(firstaddr, lastaddr - firstaddr);
 
-	/* Now read it all in. */
-	for (i = 0; i < hdr->e_shnum; i++) {
-		if (shdr[i].sh_addr == 0 || shdr[i].sh_type == SHT_NOBITS)
-			continue;
-		if (kern_pread(ef->fd, (vm_offset_t)shdr[i].sh_addr,
-		    shdr[i].sh_size, (off_t)shdr[i].sh_offset) != 0) {
+	/* Figure section with the lowest file offset we haven't loaded yet. */
+	for (cshdr = NULL; /* none */; /* none */)
+	{
+		/*
+		 * Find next section to load. The complexity of this loop is
+		 * O(n^2), but with  the number of sections being typically
+		 * small, we do not care.
+		 */
+		lshdr = cshdr;
+
+		for (i = 0; i < hdr->e_shnum; i++) {
+			if (shdr[i].sh_addr == 0 ||
+			    shdr[i].sh_type == SHT_NOBITS)
+				continue;
+			/* Skip sections that were loaded already. */
+			if (lshdr != NULL &&
+			    lshdr->sh_offset >= shdr[i].sh_offset)
+				continue;
+			/* Find section with smallest offset. */
+			if (cshdr == lshdr ||
+			    cshdr->sh_offset > shdr[i].sh_offset)
+				cshdr = &shdr[i];
+		}
+
+		if (cshdr == lshdr)
+			break;
+
+		if (kern_pread(ef->fd, (vm_offset_t)cshdr->sh_addr,
+		    cshdr->sh_size, (off_t)cshdr->sh_offset) != 0) {
 			printf("\nelf" __XSTRING(__ELF_WORD_SIZE)
 			    "_obj_loadimage: read failed\n");
 			goto out;