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https://github.com/RGBCube/serenity
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ad9e674fa0
This is a prerequisite for upstreaming our LLVM patches, as our current hack forcing `-ftls-model=initial-exec` in the Clang driver is not acceptable upstream. Currently, our kernel-managed TLS implementation limits us to only having a single block of storage for all thread-local variables that's initialized at load time. This PR merely implements the dynamic TLS interface (`__tls_get_addr` and TLSDESC) on top of our static TLS infrastructure. The current model's limitations still stand: - a single static TLS block is reserved at load time, `dlopen()`-ing shared libraries that define thread-local variables might cause us to run out of space. - the initial TLS image is not changeable post-load, so `dlopen()`-ing libraries with non-zero-initialized TLS variables is not supported. The way we repurpose `ti_module` to mean "offset within static TLS block" instead of "module index" is not ABI-compliant.
63 lines
2.4 KiB
ArmAsm
63 lines
2.4 KiB
ArmAsm
/*
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* Copyright (c) 2023, Daniel Bertalan <dani@danielbertalan.dev>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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// This file implements the runtime components of the AArch64 TLSDESC ABI,
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// which is used when accessing thread-local variables which might not be
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// stored in the static TLS block (global-dynamic and local-dynamic access
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// models). Compilers default to this when creating shared libraries, as they
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// may be loaded after program startup by `dlopen()`.
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//
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// Each referenced thread-local symbol is associated with a descriptor:
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//
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// struct TlsDescriptor {
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// size_t (*resolver)(TlsDescriptor*);
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// union {
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// size_t tpoff; // for static TLS
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// struct {
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// size_t module_id;
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// size_t module_offset;
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// } *dynamic; // for dynamic TLS, not yet implemented
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// };
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// };
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//
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// The resolver takes a pointer to the descriptor as an argument and returns
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// the symbol's offset to the thread pointer (tpidr_el1). The second field of
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// the descriptor is an implementation-defined value which the resolver uses to
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// identify the symbol.
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//
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// Thus, the address of a thread-local variable is retrieved as follows:
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//
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// &var = thread_pointer + descriptor.resolver(&descriptor);
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//
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// The two essential types of resolver functions are:
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//
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// - `__tlsdesc_static`: If the variable is located in the static TLS block,
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// its thread pointer offset is a load-time constant, which can be stored in
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// the descriptor. This function simply returns that.
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//
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// - `tlsdesc_dynamic`: Looks up a variable by its module ID and module offset.
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// This is used if the TLS block is allocated separately, so might have a
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// different thread pointer offset for each thread. This works similarly to
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// the traditional TLS ABI's __tls_get_addr function. Not yet implemented in
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// SerenityOS.
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//
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// The TLSDESC format strives to make the code sequence for thread-local
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// variable access as short as possible, hence the resolver functions follow a
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// special calling convention: they must not clobber any registers. To ensure
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// that even the usually volatile registers are saved off, we need to implement
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// the resolvers in assembly.
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// size_t __tlsdesc_static(TlsDescriptor* desc)
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// {
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// return desc->tpoff;
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// }
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.p2align 4
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.globl __tlsdesc_static
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.hidden __tlsdesc_static
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.type __tlsdesc_static,@function
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__tlsdesc_static:
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ldr x0, [x0, #8]
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ret
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