Integer overflow in `std.process.getBaseAddress` when called from a shared library linked to a C program.

[kcbanner]

Zig Version

0.11.0-dev.3893+0783dc87f

Steps to Reproduce and Observed Behavior

I discovered this while writing some standalone tests for the DWARF unwinder.

const std = @import("std");

pub fn build(b: *std.Build) void {
    const target = b.standardTargetOptions(.{});
    const optimize = b.standardOptimizeOption(.{});

    const shared_lib = b.addSharedLibrary(.{
        .name = "shared_lib",
        .root_source_file = .{ .path = "src/shared_lib.zig" },
        .target = target,
        .optimize = optimize,
    });

    const exe = b.addExecutable(.{
        .name = "test",
        .target = target,
        .optimize = optimize,
    });
    exe.addCSourceFile("src/main.c", &[_][]const u8{"-std=c99"});
    exe.linkLibrary(shared_lib);
    exe.linkLibC();

    const run_cmd = b.addRunArtifact(exe);
    run_cmd.step.dependOn(b.getInstallStep());

    const run_step = b.step("run", "Run the app");
    run_step.dependOn(&run_cmd.step);
}

main.c

#include <assert.h>
#include <stdio.h>

#include <stdint.h>
size_t getBaseAddress();

int main(int argc, char **argv) {
    size_t base_address = getBaseAddress();
    assert(base_address != 0);
    printf("base_address: %x\n", base_address);
}

shared_lib.zig

const std = @import("std");

export fn getBaseAddress() usize {
    _ = std.debug.getSelfDebugInfo() catch @panic("failed to openSelfDebugInfo");
    return std.process.getBaseAddress();
}

$ zig build run
thread 37113 panic: integer overflow

The issue is that phdr is 0 when called in this context (zig shared lib that was linked with a C program).

        .linux => {
            const base = os.system.getauxval(std.elf.AT_BASE);
            if (base != 0) {
                return base;
            }
            const phdr = os.system.getauxval(std.elf.AT_PHDR);
            return phdr - @sizeOf(std.elf.Ehdr);
        },

This also occurs if the main program is a zig program, which links a c shared lib, that links a zig shared lib that calls this function.

If you add shared_lib.linkLibC(); the problem goes away.

The problem doesn't happen if the main program is a zig program.

Expected Behavior

This function should not crash and return a valid result.