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rGc969310c992a: csu: Implement _start using as to satisfy unwinders on x86_64

Summary

The right unwinding stop indicator should be CFI-undefined PC.
https://dwarfstd.org/doc/Dwarf3.pdf - page 118:
If a Return Address register is defined in the virtual unwind table,
and its rule is undefined (for example, by DW_CFA_undefined), then
there is no return address and no call address, and the virtual
unwind of stack activations is complete.

This requires the crt code be built with unwind tables, for that remove
-fno-asynchronous-unwind-tables to enable unwind tables generation.

PR: 241562, 246322, 246537

Test Plan

Not tested with GCRT

Diff Detail

Repository

rG FreeBSD src repository

Lint

Lint Passed

Unit

No Test Coverage

Build Status

Buildable 52323
Build 49214: arc lint + arc unit

Event Timeline

dchagin created this revision.Jun 27 2023, 11:36 PM

Herald added a subscriber: imp. · View Herald TranscriptJun 27 2023, 11:36 PM

dchagin requested review of this revision.Jun 27 2023, 11:36 PM

Harbormaster completed remote builds in B52307: Diff 123875.Jun 27 2023, 11:36 PM

dchagin mentioned this in D40623: csu: Add a stop indicator to _start to satisfy unwinders on aarch64.Jun 27 2023, 11:39 PM

Please add the comments, mostly by copying/pasting the C statements for calculating each __libc_start1 arg. Also please add a comment with _start() prototype.

GCRT is the gcrt1.o compilation, it is a special kind of crt1.o intended for (almost dead) sample-based userspace profiling, AKA cc -pg. This is where profiling libraries like libc_p.a were needed.

lib/csu/amd64/crt1_s.S
30	No need to add this to new files
45	Sorry, but please keep this in .c.

Done, thank you

Harbormaster completed remote builds in B52310: Diff 123889.Jun 28 2023, 10:29 AM

dchagin marked an inline comment as done.Jun 28 2023, 10:31 AM

dchagin added inline comments.

lib/csu/amd64/crt1_s.S
45	whoops, Im missed that __progname is a pointer to an ampty string

In D40780#927982, @kib wrote:

Please add the comments, mostly by copying/pasting the C statements for calculating each __libc_start1 arg. Also please add a comment with _start() prototype.

GCRT is the gcrt1.o compilation, it is a special kind of crt1.o intended for (almost dead) sample-based userspace profiling, AKA cc -pg. This is where profiling libraries like libc_p.a were needed.

Thank you, it looks like with GCRT test compiles and even runs,
however Im a bit confused by eprol: - seems that int3 at the end of _start in GCRT case should be avoided?

kib added inline comments.Jun 28 2023, 2:02 PM

lib/csu/amd64/crt1_s.S
45	Still, I suggest to keep crt1_c.c and have environ and __progname defined there.
54
55	Remove this blank comment line
56	Add a line with register names for args like `* %rdi %rsi`
90	int3 after the `__libc_start1_gcrt` makes the same sense as after `__libc_start1`