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[BZ 18034][AArch64] Lazy TLSDESC relocation data race fix
Lazy TLSDESC initialization needs to be synchronized with concurrent TLS accesses. The TLS descriptor contains a function pointer (entry) and an argument that is accessed from the entry function. With lazy initialization the first call to the entry function updates the entry and the argument to their final value. A final entry function must make sure that it accesses an initialized argument, this needs synchronization on systems with weak memory ordering otherwise the writes of the first call can be observed out of order. There are at least two issues with the current code: tlsdesc.c (i386, x86_64, arm, aarch64) uses volatile memory accesses on the write side (in the initial entry function) instead of C11 atomics. And on systems with weak memory ordering (arm, aarch64) the read side synchronization is missing from the final entry functions (dl-tlsdesc.S). This patch only deals with aarch64. * Write side: Volatile accesses were replaced with C11 relaxed atomics, and a release store was used for the initialization of entry so the read side can synchronize with it. * Read side: TLS access generated by the compiler and an entry function code is roughly ldr x1, [x0] // load the entry blr x1 // call it entryfunc: ldr x0, [x0,#8] // load the arg ret Various alternatives were considered to force the ordering in the entry function between the two loads: (1) barrier entryfunc: dmb ishld ldr x0, [x0,#8] (2) address dependency (if the address of the second load depends on the result of the first one the ordering is guaranteed): entryfunc: ldr x1,[x0] and x1,x1,#8 orr x1,x1,#8 ldr x0,[x0,x1] (3) load-acquire (ARMv8 instruction that is ordered before subsequent loads and stores) entryfunc: ldar xzr,[x0] ldr x0,[x0,#8] Option (1) is the simplest but slowest (note: this runs at every TLS access), options (2) and (3) do one extra load from [x0] (same address loads are ordered so it happens-after the load on the call site), option (2) clobbers x1 which is problematic because existing gcc does not expect that, so approach (3) was chosen. A new _dl_tlsdesc_return_lazy entry function was introduced for lazily relocated static TLS, so non-lazy static TLS can avoid the synchronization cost. [BZ #18034] * sysdeps/aarch64/dl-tlsdesc.h (_dl_tlsdesc_return_lazy): Declare. * sysdeps/aarch64/dl-tlsdesc.S (_dl_tlsdesc_return_lazy): Define. (_dl_tlsdesc_undefweak): Guarantee TLSDESC entry and argument load-load ordering using ldar. (_dl_tlsdesc_dynamic): Likewise. (_dl_tlsdesc_return_lazy): Likewise. * sysdeps/aarch64/tlsdesc.c (_dl_tlsdesc_resolve_rela_fixup): Use relaxed atomics instead of volatile and synchronize with release store. (_dl_tlsdesc_resolve_hold_fixup): Use relaxed atomics instead of volatile. * elf/tlsdeschtab.h (_dl_tlsdesc_resolve_early_return_p): Likewise.
This commit is contained in:
Szabolcs Nagy
parent
2a8c2c7b33
commit
08325735c2
15
ChangeLog
15
ChangeLog
@ -1,3 +1,18 @@
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2015-06-17 Szabolcs Nagy <szabolcs.nagy@arm.com>
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[BZ #18034]
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* sysdeps/aarch64/dl-tlsdesc.h (_dl_tlsdesc_return_lazy): Declare.
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* sysdeps/aarch64/dl-tlsdesc.S (_dl_tlsdesc_return_lazy): Define.
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(_dl_tlsdesc_undefweak): Guarantee TLSDESC entry and argument load-load
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ordering using ldar.
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(_dl_tlsdesc_dynamic): Likewise.
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(_dl_tlsdesc_return_lazy): Likewise.
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* sysdeps/aarch64/tlsdesc.c (_dl_tlsdesc_resolve_rela_fixup): Use
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relaxed atomics instead of volatile and synchronize with release store.
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(_dl_tlsdesc_resolve_hold_fixup): Use relaxed atomics instead of
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volatile.
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* elf/tlsdeschtab.h (_dl_tlsdesc_resolve_early_return_p): Likewise.
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2015-06-15 Andrew Senkevich <andrew.senkevich@intel.com>
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* sysdeps/unix/sysv/linux/x86_64/libmvec.abilist: New symbols added.
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14
NEWS
14
NEWS
@ -15,13 +15,13 @@ Version 2.22
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17581, 17588, 17596, 17620, 17621, 17628, 17631, 17692, 17711, 17715,
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17776, 17779, 17792, 17836, 17912, 17916, 17930, 17932, 17944, 17949,
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17964, 17965, 17967, 17969, 17978, 17987, 17991, 17996, 17998, 17999,
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18007, 18019, 18020, 18029, 18030, 18032, 18036, 18038, 18039, 18042,
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18043, 18046, 18047, 18049, 18068, 18080, 18093, 18100, 18104, 18110,
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18111, 18116, 18125, 18128, 18138, 18185, 18196, 18197, 18206, 18210,
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18211, 18217, 18220, 18221, 18234, 18244, 18247, 18287, 18319, 18324,
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18333, 18346, 18397, 18409, 18410, 18412, 18418, 18422, 18434, 18444,
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18468, 18469, 18470, 18479, 18483, 18495, 18496, 18497, 18498, 18507,
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18512, 18519, 18520, 18522, 18527, 18528, 18529, 18530.
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18007, 18019, 18020, 18029, 18030, 18032, 18034, 18036, 18038, 18039,
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18042, 18043, 18046, 18047, 18049, 18068, 18080, 18093, 18100, 18104,
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18110, 18111, 18116, 18125, 18128, 18138, 18185, 18196, 18197, 18206,
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18210, 18211, 18217, 18220, 18221, 18234, 18244, 18247, 18287, 18319,
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18324, 18333, 18346, 18397, 18409, 18410, 18412, 18418, 18422, 18434,
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18444, 18468, 18469, 18470, 18479, 18483, 18495, 18496, 18497, 18498,
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18507, 18512, 18519, 18520, 18522, 18527, 18528, 18529, 18530.
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* Cache information can be queried via sysconf() function on s390 e.g. with
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_SC_LEVEL1_ICACHE_SIZE as argument.
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@ -20,6 +20,8 @@
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#ifndef TLSDESCHTAB_H
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# define TLSDESCHTAB_H 1
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#include <atomic.h>
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# ifdef SHARED
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# include <inline-hashtab.h>
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@ -138,17 +140,17 @@ _dl_make_tlsdesc_dynamic (struct link_map *map, size_t ti_offset)
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static int
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_dl_tlsdesc_resolve_early_return_p (struct tlsdesc volatile *td, void *caller)
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{
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if (caller != td->entry)
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if (caller != atomic_load_relaxed (&td->entry))
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return 1;
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__rtld_lock_lock_recursive (GL(dl_load_lock));
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if (caller != td->entry)
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if (caller != atomic_load_relaxed (&td->entry))
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{
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__rtld_lock_unlock_recursive (GL(dl_load_lock));
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return 1;
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}
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td->entry = _dl_tlsdesc_resolve_hold;
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atomic_store_relaxed (&td->entry, _dl_tlsdesc_resolve_hold);
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return 0;
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}
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@ -79,6 +79,29 @@ _dl_tlsdesc_return:
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cfi_endproc
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.size _dl_tlsdesc_return, .-_dl_tlsdesc_return
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/* Same as _dl_tlsdesc_return but with synchronization for
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lazy relocation.
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Prototype:
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_dl_tlsdesc_return_lazy (tlsdesc *) ;
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*/
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.hidden _dl_tlsdesc_return_lazy
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.global _dl_tlsdesc_return_lazy
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.type _dl_tlsdesc_return_lazy,%function
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cfi_startproc
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.align 2
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_dl_tlsdesc_return_lazy:
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/* The ldar here happens after the load from [x0] at the call site
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(that is generated by the compiler as part of the TLS access ABI),
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so it reads the same value (this function is the final value of
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td->entry) and thus it synchronizes with the release store to
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td->entry in _dl_tlsdesc_resolve_rela_fixup ensuring that the load
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from [x0,#8] here happens after the initialization of td->arg. */
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ldar xzr, [x0]
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ldr x0, [x0, #8]
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RET
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cfi_endproc
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.size _dl_tlsdesc_return_lazy, .-_dl_tlsdesc_return_lazy
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/* Handler for undefined weak TLS symbols.
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Prototype:
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_dl_tlsdesc_undefweak (tlsdesc *);
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@ -96,6 +119,13 @@ _dl_tlsdesc_return:
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_dl_tlsdesc_undefweak:
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str x1, [sp, #-16]!
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cfi_adjust_cfa_offset(16)
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/* The ldar here happens after the load from [x0] at the call site
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(that is generated by the compiler as part of the TLS access ABI),
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so it reads the same value (this function is the final value of
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td->entry) and thus it synchronizes with the release store to
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td->entry in _dl_tlsdesc_resolve_rela_fixup ensuring that the load
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from [x0,#8] here happens after the initialization of td->arg. */
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ldar xzr, [x0]
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ldr x0, [x0, #8]
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mrs x1, tpidr_el0
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sub x0, x0, x1
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@ -152,6 +182,13 @@ _dl_tlsdesc_dynamic:
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stp x3, x4, [sp, #32+16*1]
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mrs x4, tpidr_el0
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/* The ldar here happens after the load from [x0] at the call site
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(that is generated by the compiler as part of the TLS access ABI),
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so it reads the same value (this function is the final value of
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td->entry) and thus it synchronizes with the release store to
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td->entry in _dl_tlsdesc_resolve_rela_fixup ensuring that the load
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from [x0,#8] here happens after the initialization of td->arg. */
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ldar xzr, [x0]
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ldr x1, [x0,#8]
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ldr x0, [x4]
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ldr x3, [x1,#16]
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@ -45,6 +45,9 @@ struct tlsdesc_dynamic_arg
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extern ptrdiff_t attribute_hidden
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_dl_tlsdesc_return (struct tlsdesc *);
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extern ptrdiff_t attribute_hidden
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_dl_tlsdesc_return_lazy (struct tlsdesc *);
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extern ptrdiff_t attribute_hidden
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_dl_tlsdesc_undefweak (struct tlsdesc *);
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@ -25,6 +25,7 @@
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#include <dl-tlsdesc.h>
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#include <dl-unmap-segments.h>
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#include <tlsdeschtab.h>
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#include <atomic.h>
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/* The following functions take an entry_check_offset argument. It's
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computed by the caller as an offset between its entry point and the
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@ -39,11 +40,15 @@
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void
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attribute_hidden
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_dl_tlsdesc_resolve_rela_fixup (struct tlsdesc volatile *td,
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struct link_map *l)
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_dl_tlsdesc_resolve_rela_fixup (struct tlsdesc *td, struct link_map *l)
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{
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const ElfW(Rela) *reloc = td->arg;
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const ElfW(Rela) *reloc = atomic_load_relaxed (&td->arg);
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/* After GL(dl_load_lock) is grabbed only one caller can see td->entry in
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initial state in _dl_tlsdesc_resolve_early_return_p, other concurrent
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callers will return and retry calling td->entry. The updated td->entry
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synchronizes with the single writer so all read accesses here can use
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relaxed order. */
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if (_dl_tlsdesc_resolve_early_return_p
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(td, (void*)(D_PTR (l, l_info[ADDRIDX (DT_TLSDESC_PLT)]) + l->l_addr)))
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return;
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@ -86,8 +91,10 @@ _dl_tlsdesc_resolve_rela_fixup (struct tlsdesc volatile *td,
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if (!sym)
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{
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td->arg = (void*) reloc->r_addend;
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td->entry = _dl_tlsdesc_undefweak;
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atomic_store_relaxed (&td->arg, (void *) reloc->r_addend);
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/* This release store synchronizes with the ldar acquire load
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instruction in _dl_tlsdesc_undefweak. */
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atomic_store_release (&td->entry, _dl_tlsdesc_undefweak);
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}
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else
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{
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@ -96,16 +103,22 @@ _dl_tlsdesc_resolve_rela_fixup (struct tlsdesc volatile *td,
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# else
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if (!TRY_STATIC_TLS (l, result))
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{
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td->arg = _dl_make_tlsdesc_dynamic (result, sym->st_value
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void *p = _dl_make_tlsdesc_dynamic (result, sym->st_value
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+ reloc->r_addend);
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td->entry = _dl_tlsdesc_dynamic;
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atomic_store_relaxed (&td->arg, p);
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/* This release store synchronizes with the ldar acquire load
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instruction in _dl_tlsdesc_dynamic. */
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atomic_store_release (&td->entry, _dl_tlsdesc_dynamic);
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}
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else
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# endif
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{
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td->arg = (void*) (sym->st_value + result->l_tls_offset
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void *p = (void*) (sym->st_value + result->l_tls_offset
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+ reloc->r_addend);
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td->entry = _dl_tlsdesc_return;
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atomic_store_relaxed (&td->arg, p);
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/* This release store synchronizes with the ldar acquire load
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instruction in _dl_tlsdesc_return_lazy. */
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atomic_store_release (&td->entry, _dl_tlsdesc_return_lazy);
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}
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}
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@ -120,11 +133,10 @@ _dl_tlsdesc_resolve_rela_fixup (struct tlsdesc volatile *td,
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void
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attribute_hidden
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_dl_tlsdesc_resolve_hold_fixup (struct tlsdesc volatile *td,
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void *caller)
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_dl_tlsdesc_resolve_hold_fixup (struct tlsdesc *td, void *caller)
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{
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/* Maybe we're lucky and can return early. */
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if (caller != td->entry)
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if (caller != atomic_load_relaxed (&td->entry))
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return;
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/* Locking here will stop execution until the running resolver runs
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