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linux: export getauxval when not compiling with libc #16977

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Merged
kubkon merged 4 commits into from
Sep 4, 2023
Merged

linux: export getauxval when not compiling with libc #16977

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731c6d3
linux: export elf_aux_maybe so that libraries can call getauxval
kcbanner Aug 26, 2023
fba81cd
linux: instead of export elf_aux_maybe, export getauxval itself
kcbanner Aug 27, 2023
fb0cef8
linux: only export getauxval if not linking libc
kcbanner Aug 27, 2023
2e2d6d2
linux: fixup for platforms that don't support extern functions yet
kcbanner Aug 27, 2023
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18 changes: 16 additions & 2 deletions lib/std/os/linux.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -154,8 +154,22 @@ pub usingnamespace @import("linux/io_uring.zig");
/// Set by startup code, used by `getauxval`.
pub var elf_aux_maybe: ?[*]std.elf.Auxv = null;

/// See `std.elf` for the constants.
pub fn getauxval(index: usize) usize {
pub usingnamespace if (switch (builtin.zig_backend) {
// Calling extern functions is not yet supported with these backends
.stage2_x86_64, .stage2_aarch64, .stage2_arm, .stage2_riscv64, .stage2_sparc64 => false,
else => !builtin.link_libc,
}) struct {
/// See `std.elf` for the constants.
/// This matches the libc getauxval function.
pub extern fn getauxval(index: usize) usize;
comptime {
@export(getauxvalImpl, .{ .name = "getauxval", .linkage = .Weak });
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@kubkon kubkon Sep 2, 2023

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As I understand, we are exporting it as weak because we expect it to be overriden by a different version in an exe/dso that has getauxval hooked up into startup routine? What if we're building an exe and link with a dso? Which version takes precedence then? Do I even understand the problem correctly here?

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@kcbanner kcbanner Sep 4, 2023 •
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As I understand, we are exporting it as weak because we expect it to be overriden by a different version in an exe/dso that has getauxval hooked up into startup routine?

Yes, exactly. The intention is that the version used is the one that references the elf_aux_maybe initialized by the startup code.

What if we're building an exe and link with a dso? Which version takes precedence then? Do I even understand the problem correctly here?

My assumption had been that the version in the exe would take precedence, but if this is not true in all cases then this solution won't be adequate.

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@kubkon kubkon Sep 4, 2023

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Gotcha thanks!

}
} else struct {
pub const getauxval = getauxvalImpl;
};

fn getauxvalImpl(index: usize) callconv(.C) usize {
const auxv = elf_aux_maybe orelse return 0;
var i: usize = 0;
while (auxv[i].a_type != std.elf.AT_NULL) : (i += 1) {
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