From 6991a233adb1437db6ee2883acc2472843a1ddb6 Mon Sep 17 00:00:00 2001
From: Freya Murphy <freya@freyacat.org>
Date: Mon, 30 Sep 2024 15:10:16 -0400
Subject: [PATCH] fix elf section padding

---
 mld/link.c | 48 +++++++++++++++++++++++++++++++-----------------
 mld/link.h |  5 ++---
 2 files changed, 33 insertions(+), 20 deletions(-)

diff --git a/mld/link.c b/mld/link.c
index e9a886c..88b295e 100644
--- a/mld/link.c
+++ b/mld/link.c
@@ -125,7 +125,18 @@ static int relocate_segment_name(struct linker *linker, const char *name)
 				if (segtab_ent_push(ent, seg))
 					return M_ERROR;
 			} else {
-				// else create a new segment
+				// update vaddr to be page aligned
+				uint32_t m = seg->new_vaddr % SEC_ALIGN;
+				if (m) {
+					uint32_t add = SEC_ALIGN - m;
+					seg->new_vaddr += add;
+					if (B32(seg->phdr->p_flags) & PF_X)
+						linker->text_vaddr += add;
+					else
+						linker->data_vaddr += add;
+				}
+
+                                // else create a new segment
 				if (segtab_push(&linker->segments, NULL, seg))
 					return M_ERROR;
 			}
@@ -536,19 +547,21 @@ static void update_offsets(struct linker *linker)
 	linker->ehdr.e_phoff = B32(ptr);
 	ptr += linker->phdr_len * sizeof(Elf32_Phdr);
 
-	// section padding
-	{
-		uint32_t mod = ptr % SEC_ALIGN;
-		if (mod != 0)
-			linker->secalign = (SEC_ALIGN - mod);
-		else
-			 linker->secalign = 0;
-		ptr += linker->secalign;
-	}
-
 	// sections
 	for (uint32_t i = 0; i < linker->segments.len; i++) {
 		struct segment_table_entry *ent = &linker->segments.entries[i];
+
+		// section padding
+		{
+			uint32_t m = ptr % SEC_ALIGN;
+			if (m) {
+				uint32_t add = SEC_ALIGN - m;
+				ptr += add;
+				ent->off = ptr;
+				ent->padding = add;
+			}
+		}
+
 		uint32_t idx = i + 1;
 		uint32_t size = segtab_ent_size(ent);
 		linker->phdr[i].p_offset = B32(ptr);
@@ -604,15 +617,16 @@ static int write_file(struct linker *linker)
 	// phdr
 	res |= fwrite(linker->phdr, sizeof(Elf32_Phdr), linker->phdr_len, out);
 
-	// section padding
-	for (uint32_t i = 0; i < linker->secalign; i++) {
-		uint8_t zero = 0;
-		res |= fwrite(&zero, 1, 1, out);
-	}
-
 	// sections
 	for (uint32_t i = 0; i < linker->segments.len; i++) {
 		struct segment_table_entry *ent = &linker->segments.entries[i];
+		// section padding
+		{
+			for (uint32_t i = 0; i < ent->padding; i++) {
+				uint8_t zero = 0;
+				res |= fwrite(&zero, 1, 1, out);
+			}
+		}
 		for (uint32_t j = 0; j < ent->len; j++) {
 			struct segment *seg = ent->parts[j];
 			res |= fwrite(seg->bytes, 1, seg->size, out);
diff --git a/mld/link.h b/mld/link.h
index 6c3b714..435a8d5 100644
--- a/mld/link.h
+++ b/mld/link.h
@@ -170,6 +170,8 @@ struct segment_table_entry {
 	uint32_t vaddr;
 	// weak segment pointers. we do not own these!!!
 	struct segment **parts;
+	// section padding
+	uint32_t padding;
 };
 
 int segtab_ent_init(struct segment_table_entry *ent);
@@ -273,9 +275,6 @@ struct linker {
 	uint32_t strtab_shidx;
 	uint32_t shstrtab_shidx;
 
-	// section alignment after phdr bytes
-	uint32_t secalign;
-
 	// all segments
 	struct segment_table segments;
 };