Commit 58c84c59c000c341d7abcf98def0cd13d0e1630e | zig

author	Matthew Lugg <mlugg@mlugg.co.uk> 2026-08-13 21:10:56+01:00
committer	Matthew Lugg <mlugg@mlugg.co.uk> 2026-08-16 10:37:19+01:00
log	58c84c59c000c341d7abcf98def0cd13d0e1630e
tree	7b0c928593b8d663c0cd5f593d111ad0404c48eb
parent	99e54fc4209af72eb34b7d32bc5efd28a2a7b1ba
signature	Commit is signed but in an unrecognized format.

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.

1 files changed, 54 insertions(+), 17 deletions(-)

src
- link
  - Elf2.zig

src/link/Elf2.zig+54-17

 @@ -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 (plt.got_plt) |got_plt| {
-            if (elf.options.z_now) elf.ni.data_rel_ro else elf.ni.data,
+            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, .{
-        elf.shndx.plt = try elf.addSection(elf.ni.text, .{
+                .name = ".plt",
-            .name = ".plt",
+                .type = .PROGBITS,
-            .type = .PROGBITS,
+                .flags = .{ .ALLOC = true, .EXECINSTR = true },
-            .flags = .{
+                .size = plt.entry_size * plt.header_entries,
-                .ALLOC = true,
+                .addralign = plt.@"align",
-                .EXECINSTR = true,
+                .node_align = node_block_align,
-                .WRITE = plt.got_plt == null,
+            });
-            },
+        } else {
-            .size = plt.entry_size * plt.header_entries,
+            elf.shndx.plt = try elf.addSection(elf.phdrs.items[phndx.plt], .{
-            .addralign = plt.@"align",
+                .name = ".plt",
-            .node_align = node_block_align,
+                .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 },