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posix_spawn_file_actions_addchdir_np lookup doesn't work on linux musl #99740
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That used to call I do see |
Huh, thanks for looking at this. That's really weird then. |
I try this rust code and c code on musl and glibc linux system. #![feature(linkage)]
use std::ffi::c_void;
extern "C" {
#[linkage = "extern_weak"]
static posix_spawn_file_actions_addchdir_np: *const c_void;
}
fn main() {
println!("{}", unsafe {
posix_spawn_file_actions_addchdir_np.is_null()
});
} musl:
glibc:
#include <stdio.h>
__attribute__((weak)) extern void *posix_spawn_file_actions_addchdir_np;
int main() {
printf("%p\n", &posix_spawn_file_actions_addchdir_np);
} musl:
glibc:
A blog https://maskray.me/blog/2021-04-25-weak-symbol say:
So use weak symbol and checking null is not working in this case. However the old dlsym way also don't work with musl when the elf is statically linked . Maybe a compile-time check by rustc is needed when |
Hmm. Maybe instead we could add a defined weak function that returns |
If rustc bundles musl, can it just rely on the fact that it ships with this method? |
Well, in theory musl is supposed to allow external sysroots too, but it currently uses the self-contained mode for all rust/compiler/rustc_codegen_ssa/src/back/link.rs Lines 1570 to 1573 in 0b79f75
Otherwise, we could ask the question in terms of minimum supported version -- In general, it would still be nice to have a |
Still interested in this. My understanding is that once rust-lang/libc#3068 lands and is incorporated into rustc, we'll set the minimum musl requirement to 1.2 -- and that at that point we can unconditionally assume that I did some reading on weak symbols and I think I understand how to use them, but if the above understanding is correct I think we can probably just wait. |
Now that musl has been bumped to a 1.2 requirement, I have a patch locally that unconditionally enables the function on musl, and appears to work in my testing. Need to get this libc change in and released: rust-lang/libc#3949 (I presume that's preferable to defining an extern function within the rustc source code.) |
Currently, not all libcs have the `posix_spawn_file_actions_addchdir_np` symbol available to them. So we attempt to do a weak symbol lookup for that function. But that only works if libc is a dynamic library -- with statically linked musl binaries the symbol lookup would never work, so we would never be able to use it even if the musl in use supported the symbol. Now that Rust has a minimum musl version of 1.2.3, all supported musl versions now include this symbol, so we can unconditionally expect it to be there. This symbol was added to libc in rust-lang/libc#3949 -- use it here. I couldn't find any tests for whether the posix_spawn path is used, but I've verified with cargo-nextest that this change works. This is a substantial improvement to nextest's performance with musl. On my workstation with a Ryzen 7950x, against https://github.com/clap-rs/clap at 61f5ee514f8f60ed8f04c6494bdf36c19e7a8126: Before: ``` Summary [ 1.071s] 879 tests run: 879 passed, 0 skipped ``` After: ``` Summary [ 0.392s] 879 tests run: 879 passed, 0 skipped ``` Fixes rust-lang#99740.
Currently, not all libcs have the `posix_spawn_file_actions_addchdir_np` symbol available to them. So we attempt to do a weak symbol lookup for that function. But that only works if libc is a dynamic library -- with statically linked musl binaries the symbol lookup would never work, so we would never be able to use it even if the musl in use supported the symbol. Now that Rust has a minimum musl version of 1.2.3, all supported musl versions now include this symbol, so we can unconditionally expect it to be there. This symbol was added to libc in rust-lang/libc#3949 -- use it here. I couldn't find any tests for whether the posix_spawn path is used, but I've verified with cargo-nextest that this change works. This is a substantial improvement to nextest's performance with musl. On my workstation with a Ryzen 7950x, against https://github.com/clap-rs/clap at 61f5ee514f8f60ed8f04c6494bdf36c19e7a8126: Before: ``` Summary [ 1.071s] 879 tests run: 879 passed, 0 skipped ``` After: ``` Summary [ 0.392s] 879 tests run: 879 passed, 0 skipped ``` Fixes rust-lang#99740.
Currently, not all libcs have the `posix_spawn_file_actions_addchdir_np` symbol available to them. So we attempt to do a weak symbol lookup for that function. But that only works if libc is a dynamic library -- with statically linked musl binaries the symbol lookup would never work, so we would never be able to use it even if the musl in use supported the symbol. Now that Rust has a minimum musl version of 1.2.3, all supported musl versions now include this symbol, so we can unconditionally expect it to be there. This symbol was added to libc in rust-lang/libc#3949 -- use it here. I couldn't find any tests for whether the posix_spawn path is used, but I've verified with cargo-nextest that this change works. This is a substantial improvement to nextest's performance with musl. On my workstation with a Ryzen 7950x, against https://github.com/clap-rs/clap at 61f5ee514f8f60ed8f04c6494bdf36c19e7a8126: Before: ``` Summary [ 1.071s] 879 tests run: 879 passed, 0 skipped ``` After: ``` Summary [ 0.392s] 879 tests run: 879 passed, 0 skipped ``` Fixes rust-lang#99740.
Currently, not all libcs have the `posix_spawn_file_actions_addchdir_np` symbol available to them. So we attempt to do a weak symbol lookup for that function. But that only works if libc is a dynamic library -- with statically linked musl binaries the symbol lookup would never work, so we would never be able to use it even if the musl in use supported the symbol. Now that Rust has a minimum musl version of 1.2.3, all supported musl versions now include this symbol, so we can unconditionally expect it to be there. This symbol was added to libc in rust-lang/libc#3949 -- use it here. I couldn't find any tests for whether the posix_spawn path is used, but I've verified with cargo-nextest that this change works. This is a substantial improvement to nextest's performance with musl. On my workstation with a Ryzen 7950x, against https://github.com/clap-rs/clap at 61f5ee514f8f60ed8f04c6494bdf36c19e7a8126: Before: ``` Summary [ 1.071s] 879 tests run: 879 passed, 0 skipped ``` After: ``` Summary [ 0.392s] 879 tests run: 879 passed, 0 skipped ``` Fixes rust-lang#99740.
[musl] use posix_spawn if a directory change was requested Currently, not all libcs have the `posix_spawn_file_actions_addchdir_np` symbol available to them. So we attempt to do a weak symbol lookup for that function. But that only works if libc is a dynamic library -- with statically linked musl binaries the symbol lookup would never work, so we would never be able to use it even if the musl in use supported the symbol. Now that Rust has a minimum musl version of 1.2.3, all supported musl versions now include this symbol, so we can unconditionally expect it to be there. This symbol was added to libc in rust-lang/libc#3949 -- use it here. I couldn't find any tests for whether the posix_spawn path is used, but I've verified with cargo-nextest that this change works. This is a substantial improvement to nextest's performance with musl. On my workstation with a Ryzen 7950x, against https://github.com/clap-rs/clap at 61f5ee514f8f60ed8f04c6494bdf36c19e7a8126: Before: ``` Summary [ 1.071s] 879 tests run: 879 passed, 0 skipped ``` After: ``` Summary [ 0.392s] 879 tests run: 879 passed, 0 skipped ``` Fixes rust-lang#99740. try-job: dist-various-1 try-job: dist-various-2
…bilee [musl] use posix_spawn if a directory change was requested Currently, not all libcs have the `posix_spawn_file_actions_addchdir_np` symbol available to them. So we attempt to do a weak symbol lookup for that function. But that only works if libc is a dynamic library -- with statically linked musl binaries the symbol lookup would never work, so we would never be able to use it even if the musl in use supported the symbol. Now that Rust has a minimum musl version of 1.2.3, all supported musl versions now include this symbol, so we can unconditionally expect it to be there. This symbol was added to libc in rust-lang/libc#3949 -- use it here. I couldn't find any tests for whether the posix_spawn path is used, but I've verified with cargo-nextest that this change works. This is a substantial improvement to nextest's performance with musl. On my workstation with a Ryzen 7950x, against https://github.com/clap-rs/clap at 61f5ee514f8f60ed8f04c6494bdf36c19e7a8126: Before: ``` Summary [ 1.071s] 879 tests run: 879 passed, 0 skipped ``` After: ``` Summary [ 0.392s] 879 tests run: 879 passed, 0 skipped ``` Fixes rust-lang#99740. try-job: dist-various-1 try-job: dist-various-2
Rollup merge of rust-lang#131851 - sunshowers:musl-posix, r=workingjubilee [musl] use posix_spawn if a directory change was requested Currently, not all libcs have the `posix_spawn_file_actions_addchdir_np` symbol available to them. So we attempt to do a weak symbol lookup for that function. But that only works if libc is a dynamic library -- with statically linked musl binaries the symbol lookup would never work, so we would never be able to use it even if the musl in use supported the symbol. Now that Rust has a minimum musl version of 1.2.3, all supported musl versions now include this symbol, so we can unconditionally expect it to be there. This symbol was added to libc in rust-lang/libc#3949 -- use it here. I couldn't find any tests for whether the posix_spawn path is used, but I've verified with cargo-nextest that this change works. This is a substantial improvement to nextest's performance with musl. On my workstation with a Ryzen 7950x, against https://github.com/clap-rs/clap at 61f5ee514f8f60ed8f04c6494bdf36c19e7a8126: Before: ``` Summary [ 1.071s] 879 tests run: 879 passed, 0 skipped ``` After: ``` Summary [ 0.392s] 879 tests run: 879 passed, 0 skipped ``` Fixes rust-lang#99740. try-job: dist-various-1 try-job: dist-various-2
I tried this code:
(compiled under x86_64-unknown-linux-musl)
I expected to see this happen:
The
posix_spawn
fast path is used.Instead, this happened:
Rust fell back to the fork/exec slow path. (Inspected this using gdb and strace).
This is presumably because
rust/library/std/src/sys/unix/process/process_unix.rs
Lines 442 to 447 in 6dbae3a
Meta
rustc --version --verbose
:Backtrace
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