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linker: Enable full RELRO by default

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Full RELRO is a hardening feature that makes it impossible to perform
certian attacks involving overwriting parts of the Global Offset Table
to invoke arbitrary code.

It requires all symbols to be resolved before execution of the program
starts which may have an impact on startup time. However most if
not all popular Linux distributions enable full RELRO by default for
all binaries and this does not seem to make a noticeable difference
in practice.

"Partial RELRO" is equivalent to `-z relro -z lazy`.
"Full RELRO" is equivalent to `-z relro -z now`.

LLD defaults to `-z relro -z lazy`, which means Zig's current `-z relro`
option has no effect on LLD's behavior.

The changes made by this commit are as follows:

- Document that `-z relro` is the default and add `-z norelro`.
- Pass `-z now` to LLD by default to enable full RELRO by default.
- Add `-z lazy` to disable passing `-z now`.
This commit is contained in:
Isaac Freund
2022-06-08 14:33:11 +02:00
parent 61844b6bd4
commit 3381779426
3 changed files with 21 additions and 11 deletions
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4
src/Compilation.zig
View File

@@ -763,8 +763,8 @@ pub const InitOptions = struct {
linker_z_defs: bool = false,
linker_z_origin: bool = false,
linker_z_noexecstack: bool = false,
linker_z_now: bool = false,
linker_z_relro: bool = false,
linker_z_now: bool = true,
linker_z_relro: bool = true,
linker_z_nocopyreloc: bool = false,
linker_tsaware: bool = false,
linker_nxcompat: bool = false,

10
src/link/Elf.zig
View File

@@ -1517,12 +1517,12 @@ fn linkWithLLD(self: *Elf, comp: *Compilation, prog_node: *std.Progress.Node) !v
try argv.append("noexecstack");
}
if (self.base.options.z_now) {
try argv.append("-z");
try argv.append("now");
// LLD defaults to -zlazy
try argv.append("-znow");
}
if (self.base.options.z_relro) {
try argv.append("-z");
try argv.append("relro");
if (!self.base.options.z_relro) {
// LLD defaults to -zrelro
try argv.append("-znorelro");
}
if (getLDMOption(target)) |ldm| {

18
src/main.zig
View File

@@ -434,8 +434,10 @@ const usage_build_generic =
\\ origin Indicate that the object must have its origin processed
\\ nocopyreloc Disable the creation of copy relocations
\\ noexecstack Indicate that the object requires an executable stack
\\ now Force all relocations to be processed on load
\\ relro Force all relocations to be resolved and be read-only on load
\\ now (default) Force all relocations to be processed on load
\\ lazy Don't force all relocations to be processed on load
\\ relro (default) Force all relocations to be read-only after processing
\\ norelro Don't force all relocations to be read-only after processing
\\ -dynamic Force output to be dynamically linked
\\ -static Force output to be statically linked
\\ -Bsymbolic Bind global references locally
@@ -655,8 +657,8 @@ fn buildOutputType(
var linker_z_defs = false;
var linker_z_origin = false;
var linker_z_noexecstack = false;
var linker_z_now = false;
var linker_z_relro = false;
var linker_z_now = true;
var linker_z_relro = true;
var linker_tsaware = false;
var linker_nxcompat = false;
var linker_dynamicbase = false;
@@ -1209,8 +1211,12 @@ fn buildOutputType(
linker_z_noexecstack = true;
} else if (mem.eql(u8, z_arg, "now")) {
linker_z_now = true;
} else if (mem.eql(u8, z_arg, "lazy")) {
linker_z_now = false;
} else if (mem.eql(u8, z_arg, "relro")) {
linker_z_relro = true;
} else if (mem.eql(u8, z_arg, "norelro")) {
linker_z_relro = false;
} else {
warn("unsupported linker extension flag: -z {s}", .{z_arg});
}
@@ -1691,8 +1697,12 @@ fn buildOutputType(
linker_z_noexecstack = true;
} else if (mem.eql(u8, z_arg, "now")) {
linker_z_now = true;
} else if (mem.eql(u8, z_arg, "lazy")) {
linker_z_now = false;
} else if (mem.eql(u8, z_arg, "relro")) {
linker_z_relro = true;
} else if (mem.eql(u8, z_arg, "norelro")) {
linker_z_relro = false;
} else {
warn("unsupported linker extension flag: -z {s}", .{z_arg});
}