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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
34 lines
463 B
C
34 lines
463 B
C
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/* _tsrm_ls_cache is defined here */
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#include <unistd.h>
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#include <stdio.h>
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#include <stdarg.h>
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#ifdef NO_SURPLUS
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# include "def-vars.h"
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DEF_VARS(def);
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#endif
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__thread void* _tsrm_ls_cache;
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size_t tsrm_get_ls_cache_tcb_offset(void) {
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return 0;
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}
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void zend_accel_error(int type, const char *format, ...) {
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if (type < 4) {
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va_list ap;
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va_start(ap, format);
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vprintf(format, ap);
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va_end(ap);
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}
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}
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int test(void);
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int decl(void) {
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return test();
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}
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