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starkernel: fix lidt() inline-asm constraint hazard, land -O2

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FABRIC.md item 4.5d Finding 4: lidt()'s inline asm used a register-only
("r") constraint on the idtr pointer, never telling GCC the asm
dereferences the pointee. At -O2 this let the compiler treat the
256-entry idt[] population loop and idtr_desc's field writes as dead
stores and eliminate them entirely, loading IDTR from uninitialized
stack instead of the real table -- a #GP on the first APIC timer tick
that happened to land on garbage. Same bug class as the earlier
muldiv64 fix (b43e51a): an inline-asm constraint too weak for what the
asm actually touches, invisible at -O0, live at -O2.

Fixed by switching to a memory operand ("m"(*idtr_desc)), matching how
Linux's own load_idt() is written. aarch64/riscv64 checked for the same
pattern -- neither has it, both install their vector/trap tables
entirely in hand-written .S.

Verified: all three architectures boot clean to ok>, POST Failed: 0,
identical dict-hashes across all three under -O2, zero new warnings
vs an -O0 baseline (amd64 3040/3040, aarch64 3041/3041 serial,
riscv64 3037/3037). -O2/-U_FORTIFY_SOURCE landed permanently in
COMMON_CFLAGS.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
This commit is contained in:
Robert Allan James
2026-08-11 16:57:20 -04:00
co-authored by Claude Sonnet 5
parent 065ab50240
commit a1c9fc7330
13 changed files with 54912 additions and 87 deletions
Download Patch File Download Diff File Expand all files Collapse all files
experiments/bare_metal/latest/aarch64.csv
+12 -12
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@@ -78,33 +78,33 @@ tick_number,elapsed_ns,tick_interval_ns,cache_hits_delta,bucket_hits_delta,word_
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103 29 290000 10000 0 0 256 45 24 512 512 0 0 171 65536 0 1 65536 0 0
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105 31 310000 10000 0 0 256 45 25 256 256 0 0 184 65536 0 1 65536 0 0
106 32 320000 10000 0 0 255 45 26 256 256 0 0 190 65536 0 1 65536 0 0
107 33 330000 10000 0 0 256 45 27 256 256 0 0 197 196 65536 0 1 65536 0 0
108 34 340000 10000 0 0 256 45 27 256 256 0 0 203 65536 0 1 65536 0 0
109 35 350000 10000 0 0 256 45 28 256 256 0 0 209 65536 0 1 65536 0 0
110 36 360000 10000 0 0 256 45 29 256 256 0 0 216 65536 0 1 65536 0 0