use "INSERT AFTER" in embedded-test.x

[kaspar030]

TL/DR: add INSERT AFTER .comment; to embedded-test.x so it never overrides a default linker script.

Currently, the embedded-test.x is supposed to be added to builds by adding -Tembedded-test.x to the linker args.

-Tfoo is used to specify a new default linker script. If multiple linker scripts are added using -T..., the first replaces the default script, the rest is then considered to be augmenting other linker scripts.

On embedded, there are usually other linker scripts before -Tembedded-test.x, e.g., on cortex-m, it is -Tlink.x.

Now on std, we don't usually have another linker script, and some ld/lld provided default script is used. Here, adding -Tembedded-test.x would override that default script, causing linking to fail.

For the std example, I worked around that by not using -T but adding a relative path to the in-repo embedded-test.x. That works in this repo, for this example, but requires similar action for any users. The ususal println!("cargo::rustc-link-arg-tests=-Tembedded-test.x"); doesn't work on std.

I went into a deep dive on how this works, and found out:

• for -Txxx, the linker search path is looked up
• Both GNU ld and lld parse non-objects/archives as linker scripts, but without -T, the linker search path is not used.
• Both GNU ld and lld do search the linker path if a file is added via -lfilename, and don't forcably use libfilename if filename is prefixed with :, but only GNU ld accepts linker scripts this way
• both GNU ld and lld always treat a linker script as augmenting linker script if it contains INSERT AFTER/BEFORE, even if it is added using -T.

So in order to not override the default linker script on std, I've added INSERT AFTER .comment; into embedded-test.x. In my testing, that .comment section is inserted by GNU ld and lld on linux, cortex-m, riscv and xtensa.

Signed-off-by: Kaspar Schleiser kaspar@schleiser.de

[t-moe]

Thank you for deep-diving into this. That is a nice solution.
Can you add a small comment in the linker-file, explaining the line and why its needed?

[kaspar030]

Can you add a small comment in the linker-file, explaining the line and why its needed?

Done, not sure about the wording ...

[t-moe]

Thank you.
Do you need a release now or can I wait & bundle the changes with other stuff?

[kaspar030]

Do you need a release now or can I wait & bundle the changes with other stuff?

It can wait, we can use this from our branch for now.

Thanks!

[bugadani]

In my testing, that .comment section is inserted by GNU ld and lld on linux, cortex-m, riscv and xtensa.

I don't know what you did, but naively trying to update embedded-test results in esp-hal tests failing to compile with .comment not found for insert.

[t-moe]

I don't know what you did, but naively trying to update embedded-test results in esp-hal tests failing to compile with .comment not found for insert.

can you share a failing build?

[bugadani]

You can now see the build errors e.g. at https://github.com/esp-rs/esp-hal/actions/runs/16747543756/job/47409604439?pr=3889

[t-moe]

You can now see the build errors e.g. at https://github.com/esp-rs/esp-hal/actions/runs/16747543756/job/47409604439?pr=3889

hmm, fails on xtensa only.
@kaspar030 you tested this on xtensa as well?

[kaspar030]

I think I did test on xtensa, wouldn't swear on it though. I'm on vacations for two more weeks, will deal with it then!