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We use linker relocations to fetch the TLS index and offset of _tsrm_ls_cache.
When building Opcache statically, linkers may attempt to optimize that into a
more efficient code sequence (relaxing from "General Dynamic" to "Local Exec"
model [1]). Unfortunately, linkers will fail, rather than ignore our
relocations, when they don't recognize the exact code sequence they are
expecting.
This results in errors as reported by GH-15074:
TLS transition from R_X86_64_TLSGD to R_X86_64_GOTTPOFF against
`_tsrm_ls_cache' at 0x12fc3 in section `.text' failed"
Here I take a different approach:
* Emit the exact full code sequence expected by linkers
* Extract the TLS index/offset by inspecting the linked ASM code, rather than
executing it (execution would give us the thread-local address).
* We detect when the code was relaxed, in which case we can extract the TCB
offset instead.
* This is done in a conservative way so that if the linker did something we
didn't expect, we fallback to a safer (but slower) mechanism.
One additional benefit of that is we are now able to use the Local Exec model in
more cases, in JIT'ed code. This makes non-glibc builds faster in these cases.
Closes GH-18939.
Related RFC: https://wiki.php.net/rfc/make_opcache_required.
[1] https://www.akkadia.org/drepper/tls.pdf
64 lines
2.2 KiB
C
64 lines
2.2 KiB
C
/*
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* +----------------------------------------------------------------------+
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* | Zend JIT |
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* +----------------------------------------------------------------------+
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* | Copyright (c) The PHP Group |
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* +----------------------------------------------------------------------+
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* | This source file is subject to version 3.01 of the PHP license, |
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* | that is bundled with this package in the file LICENSE, and is |
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* | available through the world-wide-web at the following url: |
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* | https://www.php.net/license/3_01.txt |
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* | If you did not receive a copy of the PHP license and are unable to |
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* | obtain it through the world-wide-web, please send a note to |
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* | license@php.net so we can mail you a copy immediately. |
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* +----------------------------------------------------------------------+
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* | Authors: Dmitry Stogov <dmitry@php.net> |
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* +----------------------------------------------------------------------+
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*/
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#include "Zend/zend_portability.h"
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#include "Zend/zend_types.h"
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#include "TSRM/TSRM.h"
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#include "zend_accelerator_debug.h"
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#include <stdint.h>
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#include <stddef.h>
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TSRMLS_CACHE_EXTERN();
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extern uint32_t _tls_index;
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extern char *_tls_start;
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extern char *_tls_end;
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zend_result zend_jit_resolve_tsrm_ls_cache_offsets(
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size_t *tcb_offset,
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size_t *module_index,
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size_t *module_offset
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) {
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/* To find offset of "_tsrm_ls_cache" in TLS segment we perform a linear scan of local TLS memory */
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/* Probably, it might be better solution */
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#ifdef _WIN64
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void ***tls_mem = ((void****)__readgsqword(0x58))[_tls_index];
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#else
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void ***tls_mem = ((void****)__readfsdword(0x2c))[_tls_index];
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#endif
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void *val = _tsrm_ls_cache;
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size_t offset = 0;
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size_t size = (char*)&_tls_end - (char*)&_tls_start;
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while (offset < size) {
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if (*tls_mem == val) {
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*module_index = _tls_index * sizeof(void*);
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*module_offset = offset;
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return SUCCESS;
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}
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tls_mem++;
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offset += sizeof(void*);
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}
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if (offset >= size) {
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zend_accel_error_noreturn(ACCEL_LOG_FATAL, "Could not enable JIT: offset >= size");
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}
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return FAILURE;
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}
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