From 58c84c59c000c341d7abcf98def0cd13d0e1630e Mon Sep 17 00:00:00 2001
From: Matthew Lugg <mlugg@mlugg.co.uk>
Date: Thu, 13 Aug 2026 21:10:56 +0100
Subject: [PATCH] Elf2: add segment for PLT on SPARC

On SPARC, because JUMP_SLOT relocations write directly to the PLT, the
PLT is required to be RWX. Conventionally, this is achieved by placing
it in the "data" segment and making that RWX instead of RW, but that
just seems unnecessarily dangerous. Instead, let's try making a segment
specifically for the PLT.
---
 src/link/Elf2.zig | 71 +++++++++++++++++++++++++++++++++++------------
 1 file changed, 54 insertions(+), 17 deletions(-)

diff --git a/src/link/Elf2.zig b/src/link/Elf2.zig
index 66e01f3571a05a8ac446c3682002255e91106120..f616f5dbaf223daa9381c6f12a48ec6d897dda80 100644
--- a/src/link/Elf2.zig
+++ b/src/link/Elf2.zig
@@ -3521,6 +3521,10 @@ fn initHeaders(
         rodata: u32,
         text: u32,
         data: u32,
+        /// On most targets this is `undefined`, but on machines where JUMP_SLOT relocations write
+        /// directly to the PLT, we place the PLT in its own segment in order to avoid making the
+        /// general data segment RWX.
+        plt: u32,
         tls: u32,
         dynamic: u32,
         relro: u32,
@@ -3552,6 +3556,10 @@ fn initHeaders(
                 defer phnum += 1;
                 break :phndx phnum;
             },
+            .plt = if (plt.got_plt == null) phndx: {
+                defer phnum += 1;
+                break :phndx phnum;
+            } else undefined,
             .tls = if (comp.config.any_non_single_threaded) phndx: {
                 defer phnum += 1;
                 break :phndx phnum;
@@ -3721,6 +3729,16 @@ fn initHeaders(
         elf.nodes.appendAssumeCapacity(.{ .segment = phndx.data });
         elf.phdrs.items[phndx.data] = elf.ni.data;
 
+        if (plt.got_plt == null) {
+            const plt_ni = try elf.mf.addLastChildNode(gpa, elf.ni.elf, .{
+                .alignment = node_block_align,
+                .moved = true,
+                .bubbles_moved = false,
+            });
+            elf.nodes.appendAssumeCapacity(.{ .segment = phndx.plt });
+            elf.phdrs.items[phndx.plt] = plt_ni;
+        }
+
         elf.ni.data_rel_ro = try elf.mf.addOnlyChildNode(gpa, elf.ni.data, .{
             // Must be at least `addr_align` for the `PT_DYNAMIC` node to be placed inside this one
             // later (if `have_dynamic_section`). Keep in sync with `elf.ni.data` alignment above.
@@ -3844,6 +3862,20 @@ fn initHeaders(
                     .@"align" = @intCast(page_align.toByteUnits()),
                 };
 
+                if (plt.got_plt == null) {
+                    const ph_plt = &phdr[phndx.plt];
+                    ph_plt.* = .{
+                        .type = .NULL,
+                        .offset = 0,
+                        .vaddr = @intCast(base_vaddr),
+                        .paddr = @intCast(base_vaddr),
+                        .filesz = 0,
+                        .memsz = 0,
+                        .flags = .{ .R = true, .W = true, .X = true },
+                        .@"align" = @intCast(page_align.toByteUnits()),
+                    };
+                }
+
                 if (comp.config.any_non_single_threaded) {
                     const ph_tls = &phdr[phndx.tls];
                     ph_tls.* = .{
@@ -4001,29 +4033,34 @@ fn initHeaders(
             .addralign = addr_align,
             .entsize = @intCast(addr_align.toByteUnits()),
         });
-        if (plt.got_plt) |got_plt| elf.shndx.got_plt = try elf.addSection(
-            if (elf.options.z_now) elf.ni.data_rel_ro else elf.ni.data,
-            .{
+        if (plt.got_plt) |got_plt| {
+            const got_plt_segment_ni = if (elf.options.z_now) elf.ni.data_rel_ro else elf.ni.data;
+            elf.shndx.got_plt = try elf.addSection(got_plt_segment_ni, .{
                 .name = ".got.plt",
                 .type = .PROGBITS,
                 .flags = .{ .WRITE = true, .ALLOC = true },
                 .size = got_plt.header_entries * elf.targetPtrSize(),
                 .addralign = addr_align,
                 .entsize = @intCast(addr_align.toByteUnits()),
-            },
-        );
-        elf.shndx.plt = try elf.addSection(elf.ni.text, .{
-            .name = ".plt",
-            .type = .PROGBITS,
-            .flags = .{
-                .ALLOC = true,
-                .EXECINSTR = true,
-                .WRITE = plt.got_plt == null,
-            },
-            .size = plt.entry_size * plt.header_entries,
-            .addralign = plt.@"align",
-            .node_align = node_block_align,
-        });
+            });
+            elf.shndx.plt = try elf.addSection(elf.ni.text, .{
+                .name = ".plt",
+                .type = .PROGBITS,
+                .flags = .{ .ALLOC = true, .EXECINSTR = true },
+                .size = plt.entry_size * plt.header_entries,
+                .addralign = plt.@"align",
+                .node_align = node_block_align,
+            });
+        } else {
+            elf.shndx.plt = try elf.addSection(elf.phdrs.items[phndx.plt], .{
+                .name = ".plt",
+                .type = .PROGBITS,
+                .flags = .{ .ALLOC = true, .WRITE = true, .EXECINSTR = true },
+                .size = plt.entry_size * plt.header_entries,
+                .addralign = plt.@"align",
+                .node_align = node_block_align,
+            });
+        }
         if (plt.plt_sec != null) elf.shndx.plt_sec = try elf.addSection(elf.ni.text, .{
             .name = ".plt.sec",
             .flags = .{ .ALLOC = true, .EXECINSTR = true },
-- 
2.54.0