118 lines
5.7 KiB
C
118 lines
5.7 KiB
C
#ifndef VLIBC_INTERNAL_START_TCB_H
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#define VLIBC_INTERNAL_START_TCB_H
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/*
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* vlibc — AUTHORITATIVE x86_64 TCB / thread-pointer layout (todo 3).
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*
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* This header defines the thread control block (TCB) layout ONCE, for the
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* whole library. Later layers reference these constants and NEVER redefine
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* them:
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*
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* #44 threading substrate: clones this layout for every new thread and
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* initializes the fenv slot per thread.
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* #45 sizes new-thread TLS from the static TLS block arithmetic below.
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* #62 ld.so: performs the dynamic-case equivalent of this bootstrap (the
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* FS-already-set guard in tls.c skips this code in that case).
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* #42 fenv: reads/writes the reserved fenv slot through the fixed
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* offsets below; the slot is per-thread lazy storage for the x87
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* control word and MXCSR.
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*
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* ABI model — ELF TLS variant II on x86_64:
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*
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* The thread pointer (TP) is delivered by the %fs segment and points
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* DIRECTLY at the TCB (this is what "variant II" means; variant I, used by
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* e.g. 32-bit x86, points at the end of the TLS data instead). Static TLS
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* data live at NEGATIVE offsets from TP (%fs:N with N < 0, the LE model);
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* the TCB fields live at positive offsets. Slot 0 is required to hold the
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* TCB's own address because __builtin_thread_pointer() reads %fs:0 — that
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* is the entire mechanism by which C code recovers TP from a register.
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*
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* TCB field map (offsets are byte offsets from TP, i.e. from %fs:0):
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*
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* TP + 0 VLIBC_TCB_SELF_OFF TCB self pointer (TP itself).
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* TP + 8 VLIBC_TCB_ERRNO_OFF per-thread errno slot: 8 bytes, errno
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* occupies the low 4. This offset is OWNED
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* by src/internal/errno.h (task 1 fixed it
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* as an ABI constant) and is consumed
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* here, never re-derived.
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* TP + 16 VLIBC_TCB_FENV_OFF per-thread fenv reservation, 8 bytes:
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* +16 VLIBC_TCB_FENV_X87CW_OFF uint16 x87 control word,
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* reset default 0x037F (double
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* precision, round-to-nearest)
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* +18 (pad, reserved)
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* +20 VLIBC_TCB_FENV_MXCSR_OFF uint32 MXCSR, reset default
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* 0x1F80 (all exceptions masked)
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* TP + 24 end of the fixed layout; VLIBC_TCB_SIZE = 24.
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*
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* The layout grows FORWARD from TP if later layers need more TCB fields;
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* the fixed slots above are never renumbered. A DTV pointer is deliberately
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* NOT reserved: the static TLS model needs no DTV (every access is
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* %fs-relative with a link-time offset; there is no __tls_get_addr). Should
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* #62's dynamic TLS ever require a DTV, add it at TP + VLIBC_TCB_SIZE —
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* glibc's variant-II convention keeps the DTV at a negative offset relative
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* to a separately allocated block; vlibc deliberately keeps the simpler
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* grow-upward model documented here.
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*
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* Static TLS placement contract (tls.c implements this; the linker bakes
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* its TPOFF arithmetic against the same formula, verified empirically with
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* GNU ld):
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*
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* - tp = base + round_up(p_memsz, p_align), with base page-aligned.
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* - the PT_TLS image is copied to [base, base + p_filesz), the BSS tail
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* [base + p_filesz, base + p_memsz) is zeroed, and the TCB occupies
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* [tp, tp + VLIBC_TCB_SIZE).
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* - TP alignment equals p_align EXACTLY (never bumped to 16): the linker
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* computes tpoff = var_offset - round_up(p_memsz, p_align), so any
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* other rounding breaks every %fs offset when p_memsz is not a
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* multiple of the runtime's alignment. When the program has no PT_TLS
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* at all, p_align is taken as 16 and the block is TCB-only.
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*
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* TCB fields only need 8-byte alignment, and the linker guarantees
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* p_align >= the alignment of every TLS object, so the exact-p_align
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* rule is always sufficient.
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*/
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#include "../internal/errno.h" /* VLIBC_TCB_ERRNO_OFF */
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#include <stddef.h>
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#include <stdint.h>
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#define VLIBC_TCB_SELF_OFF 0
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/* VLIBC_TCB_ERRNO_OFF (8) is consumed from src/internal/errno.h. */
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#define VLIBC_TCB_FENV_OFF 16
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#define VLIBC_TCB_FENV_X87CW_OFF (VLIBC_TCB_FENV_OFF + 0)
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#define VLIBC_TCB_FENV_MXCSR_OFF (VLIBC_TCB_FENV_OFF + 4)
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#define VLIBC_TCB_SIZE 24
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/*
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* New-thread TLS geometry (todo 44). All four are set by init_main_tls()
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* in src/start/tls.c and read by src/thread/tls.c (__vlibc_tls_setup):
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*
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* __vlibc_static_tls_size PT_TLS p_memsz of the main executable — the
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* one DEFAULT-VISIBILITY EXPORTED symbol of
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* the group: #62 grows it on dlopen, #45 reads
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* it when sizing new-thread TLS blocks.
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* __vlibc_static_tls_filesz PT_TLS p_filesz (.tdata part).
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* __vlibc_static_tls_align PT_TLS p_align (16 when there is no PT_TLS).
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* __vlibc_static_tls_template the PRIMAL TLS image address: the mapped
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* .tdata at p_vaddr, never the main thread's
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* LIVE TLS block (which mutates). In the
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* file-fallback path it is the main block
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* itself — the only copy that exists then.
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*
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* The three hidden ones keep their attribute on the definitions (as with
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* environ's hidden alias in src/start/environ.c); __vlibc_static_tls_size
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* deliberately carries none so it stays an exported ABI object.
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*/
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// NOLINTBEGIN(bugprone-reserved-identifier)
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extern size_t __vlibc_static_tls_size;
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extern uintptr_t __vlibc_static_tls_template;
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extern size_t __vlibc_static_tls_filesz;
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extern size_t __vlibc_static_tls_align;
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// NOLINTEND(bugprone-reserved-identifier)
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#endif /* VLIBC_INTERNAL_START_TCB_H */
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