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the zero initialization is redundant since decode_vec does its own
clearing, and it increases the risk that buggy compilers will generate
calls to memset. as long as symbols are bound at ld time, such a call
will not break anything, but it may be desirable to turn off ld-time
binding in the future.
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since 1.1.0, musl has nominally required a thread pointer to be setup.
most of the remaining code that was checking for its availability was
doing so for the sake of being usable by the dynamic linker. as of
commit 71f099cb7db821c51d8f39dfac622c61e54d794c, this is no longer
necessary; the thread pointer is now valid before any libc code
(outside of dynamic linker bootstrap functions) runs.
this commit essentially concludes "phase 3" of the "transition path
for removing lazy init of thread pointer" project that began during
the 1.1.0 release cycle.
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this allows the dynamic linker itself to run with a valid thread
pointer, which is a prerequisite for stack protector on archs where
the ssp canary is stored in TLS. it will also allow us to remove some
remaining runtime checks for whether the thread pointer is valid.
as long as the application and its libraries do not require additional
size or alignment, this early thread pointer will be kept and reused
at runtime. otherwise, a new static TLS block is allocated after
library loading has finished and the thread pointer is switched over.
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previously, the layout of the static TLS block was perturbed by the
size of the dtv; dtv size increasing from 0 to 1 perturbed both TLS
arch types, and the TLS-above-TP type's layout was perturbed by the
specific number of dtv slots (libraries with TLS). this behavior made
it virtually impossible to setup a tentative thread pointer address
before loading libraries and keep it unchanged as long as the
libraries' TLS size/alignment requirements fit.
the new code fixes the location of the dtv and pthread structure at
opposite ends of the static TLS block so that they will not move
unless size or alignment changes.
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this overhaul further reduces the amount of arch-specific code needed
by the dynamic linker and removes a number of assumptions, including:
- that symbolic function references inside libc are bound at link time
via the linker option -Bsymbolic-functions.
- that libc functions used by the dynamic linker do not require
access to data symbols.
- that static/internal function calls and data accesses can be made
without performing any relocations, or that arch-specific startup
code handled any such relocations needed.
removing these assumptions paves the way for allowing libc.so itself
to be built with stack protector (among other things), and is achieved
by a three-stage bootstrap process:
1. relative relocations are processed with a flat function.
2. symbolic relocations are processed with no external calls/data.
3. main program and dependency libs are processed with a
fully-functional libc/ldso.
reduction in arch-specific code is achived through the following:
- crt_arch.h, used for generating crt1.o, now provides the entry point
for the dynamic linker too.
- asm is no longer responsible for skipping the beginning of argv[]
when ldso is invoked as a command.
- the functionality previously provided by __reloc_self for heavily
GOT-dependent RISC archs is now the arch-agnostic stage-1.
- arch-specific relocation type codes are mapped directly as macros
rather than via an inline translation function/switch statement.
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when dlopen fails, all partially-loaded libraries need to be unmapped
and freed. any of these libraries using an rpath with $ORIGIN
expansion may have an allocated string for the expanded rpath;
previously, this string was not freed when freeing the library data
structures.
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this change hardens the dynamic linker against the possibility of
loading the wrong library due to inability to expand $ORIGIN in rpath.
hard failures such as excessively long paths or absence of /proc (when
resolving /proc/self/exe for the main executable's origin) do not stop
the path search, but memory allocation failures and any other
potentially transient failures do.
to implement this change, the meaning of the return value of
fixup_rpath function is changed. returning zero no longer indicates
that the dso's rpath string pointer is non-null; instead, the caller
needs to check. a return value of -1 indicates a failure that should
stop further path search.
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transient errors during the path search should not allow the search to
continue and possibly open the wrong file. this patch eliminates most
conditions where that could happen, but there is still a possibility
that $ORIGIN-based rpath processing will have an allocation failure,
causing the search to skip such a path. fixing this is left as a
separate task.
a small bug where overly-long path components caused an infinite loop
rather than being skipped/ignored is also fixed.
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There are two main abi variants for thread local storage layout:
(1) TLS is above the thread pointer at a fixed offset and the pthread
struct is below that. So the end of the struct is at known offset.
(2) the thread pointer points to the pthread struct and TLS starts
below it. So the start of the struct is at known (zero) offset.
Assembly code for the dynamic TLSDESC callback needs to access the
dynamic thread vector (dtv) pointer which is currently at the front
of the pthread struct. So in case of (1) the asm code needs to hard
code the offset from the end of the struct which can easily break if
the struct changes.
This commit adds a copy of the dtv at the end of the struct. New members
must not be added after dtv_copy, only before it. The size of the struct
is increased a bit, but there is opportunity for size optimizations.
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a conservative estimate of 4*sizeof(size_t) was used as the minimum
alignment for thread-local storage, despite the only requirements
being alignment suitable for struct pthread and void* (which struct
pthread already contains). additional alignment required by the
application or libraries is encoded in their headers and is already
applied.
over-alignment prevented the builtin_tls array from ever being used in
dynamic-linked programs on 64-bit archs, thereby requiring allocation
at startup even in programs with no TLS of their own.
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the memory model we use internally for atomics permits plain loads of
values which may be subject to concurrent modification without
requiring that a special load function be used. since a compiler is
free to make transformations that alter the number of loads or the way
in which loads are performed, the compiler is theoretically free to
break this usage. the most obvious concern is with atomic cas
constructs: something of the form tmp=*p;a_cas(p,tmp,f(tmp)); could be
transformed to a_cas(p,*p,f(*p)); where the latter is intended to show
multiple loads of *p whose resulting values might fail to be equal;
this would break the atomicity of the whole operation. but even more
fundamental breakage is possible.
with the changes being made now, objects that may be modified by
atomics are modeled as volatile, and the atomic operations performed
on them by other threads are modeled as asynchronous stores by
hardware which happens to be acting on the request of another thread.
such modeling of course does not itself address memory synchronization
between cores/cpus, but that aspect was already handled. this all
seems less than ideal, but it's the best we can do without mandating a
C11 compiler and using the C11 model for atomics.
in the case of pthread_once_t, the ABI type of the underlying object
is not volatile-qualified. so we are assuming that accessing the
object through a volatile-qualified lvalue via casts yields volatile
access semantics. the language of the C standard is somewhat unclear
on this matter, but this is an assumption the linux kernel also makes,
and seems to be the correct interpretation of the standard.
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the new DT_RUNPATH semantics for search order are always used, and
since binutils had always set both DT_RPATH and DT_RUNPATH when the
latter was used, processing only DT_RPATH worked fine. however, recent
binutils has stopped generating DT_RPATH when DT_RUNPATH is used,
which broke support for this feature completely.
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this allows most code to assume it has already been saved, and is a
prerequisite for upcoming changes for arm atomic/tls operations.
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when the dynamic loader is disabled, dlopen fails correctly but dlerror
did not return a human readable error string like it should have.
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previously passing an empty string for name resulted in failure, as
expected, but only after spurious syscalls, and it produced confusing
errno values (and thus dlerror strings).
in addition to dlopen calls, this issue affected use of LD_PRELOAD
with trailing whitespace or colon characters.
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this issue caused the address of functions in shared libraries to
resolve to their PLT thunks in the main program rather than their
correct addresses. it was observed causing crashes, though the
mechanism of the crash was not thoroughly investigated. since the
issue is very subtle, it calls for some explanation:
on all well-behaved archs, GOT entries that belong to the PLT use a
special relocation type, typically called JMP_SLOT, so that the
dynamic linker can avoid having the jump destinations for the PLT
resolve to PLT thunks themselves (they also provide a definition for
the symbol, which must be used whenever the address of the function is
taken so that all DSOs see the same address).
however, the traditional mips PIC ABI lacked such a JMP_SLOT
relocation type, presumably because, due to the way PIC works, the
address of the PLT thunk was never needed and could always be ignored.
prior to commit adf94c19666e687a728bbf398f9a88ea4ea19996, the mips
version of reloc.h contained a hack that caused all symbol lookups to
be treated like JMP_SLOT, inhibiting undefined symbols from ever being
used to resolve symbolic relocations. this hack goes all the way back
to commit babf820180368f00742ec65b2050a82380d7c542, when the mips
dynamic linker was first made usable.
during the recent refactoring to eliminate arch-specific relocation
processing (commit adf94c19666e687a728bbf398f9a88ea4ea19996), this
hack was overlooked and no equivalent functionality was provided in
the new code.
fixing the problem is not as simple as adding back an equivalent hack,
since there is now also a "non-PIC ABI" that can be used for the main
executable, which actually does use a PLT. the closest thing to
official documentation I could find for this ABI is nonpic.txt,
attached to Message-ID: 20080701202236.GA1534@caradoc.them.org, which
can be found in the gcc mailing list archives and elsewhere. per this
document, undefined symbols corresponding to PLT thunks have the
STO_MIPS_PLT bit set in the symbol's st_other field. thus, I have
added an arch-specific rule for mips, applied at the find_sym level
rather than the relocation level, to reject undefined symbols with the
STO_MIPS_PLT bit clear.
the previous hack of treating all mips relocations as JMP_SLOT-like,
rather than rejecting the unwanted symbols in find_sym, probably also
caused dlsym to wrongly return PLT thunks in place of the correct
address of a function under at least some conditions. this should now
be fixed, at least for global-scope symbol lookups.
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due to a mistake when refactoring the error printing for the dynamic
linker (commit 7c73cacd09a51a87484db5689864743e4984a84d), all messages
were suppressed and replaced by blank lines.
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such separation serves multiple purposes:
- by having the common path for __tls_get_addr alone in its own
function with a tail call to the slow case, code generation is
greatly improved.
- by having __tls_get_addr in it own file, it can be replaced on a
per-arch basis as needed, for optimization or ABI-specific purposes.
- by removing __tls_get_addr from __init_tls.c, a few bytes of code
are shaved off of static binaries (which are unlikely to use this
function unless the linker messed up).
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previously, accesses to dynamic TLS had to check two conditions before
being able to use a dtv slot: (1) that the module index was within the
bounds of the current dtv size, and (2) that the dynamic tls for the
requested module index was already installed in the dtv.
this commit changes the installation strategy so that, whenever an
attempt is made to access dynamic TLS that's not yet installed in the
dtv, the dynamic TLS for all lower-index modules is also installed.
thus it provides a new invariant: if a given module index is within
the bounds of the current dtv size, we automatically know that its TLS
is installed and directly available. the requirement that the second
condition (above) be checked is eliminated.
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this code is non-functional without further changes to link up the
arch-specific reloc types for tlsdesc and add asm implementations of
__tlsdesc_static and __tlsdesc_dynamic.
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eventually this should help making dlerror thread-safe too.
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this was one of the main instances of ugly code duplication: all archs
use basically the same types of relocations, but roughly equivalent
logic was duplicated for each arch to account for the different naming
and numbering of relocation types and variation in whether REL or RELA
records are used.
as an added bonus, both REL and RELA are now supported on all archs,
regardless of which is used by the standard toolchain.
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so far the options are --library-path and --preload which override the
corresponding environment variables, and --list which forces the
behavior of ldd even if the invocation name is not ldd. both the
two-arg form and the one-arg form using an equals sign are supported.
based loosely on a patch proposed by Rune.
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now that thread pointer is initialized always, ssp canary
initialization can be done unconditionally. this simplifies
the ldso as it does not try to detect ssp usage, and the
init function itself as it is always called exactly once.
this also merges ssp init path for shared and static linking.
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consistent use of braces in if/else structure, line length.
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record phentsize in struct dso, so the phdrs can be easily
enumerated via it. simplify all functions enumerating phdrs
to require only struct dso. also merge find_map_range and
find_dso to kernel_mapped_dso function that does both tasks
during single phdr enumeration.
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this is the first step in an overhaul aimed at greatly simplifying and
optimizing everything dealing with thread-local state.
previously, the thread pointer was initialized lazily on first access,
or at program startup if stack protector was in use, or at certain
random places where inconsistent state could be reached if it were not
initialized early. while believed to be fully correct, the logic was
fragile and non-obvious.
in the first phase of the thread pointer overhaul, support is retained
(and in some cases improved) for systems/situation where loading the
thread pointer fails, e.g. old kernels.
some notes on specific changes:
- the confusing use of libc.main_thread as an indicator that the
thread pointer is initialized is eliminated in favor of an explicit
has_thread_pointer predicate.
- sigaction no longer needs to ensure that the thread pointer is
initialized before installing a signal handler (this was needed to
prevent a situation where the signal handler caused the thread
pointer to be initialized and the subsequent sigreturn cleared it
again) but it still needs to ensure that implementation-internal
thread-related signals are not blocked.
- pthread tsd initialization for the main thread is deferred in a new
manner to minimize bloat in the static-linked __init_tp code.
- pthread_setcancelstate no longer needs special handling for the
situation before the thread pointer is initialized. it simply fails
on systems that cannot support a thread pointer, which are
non-conforming anyway.
- pthread_cleanup_push/pop now check for missing thread pointer and
nop themselves out in this case, so stdio no longer needs to avoid
the cancellable path when the thread pointer is not available.
a number of cases remain where certain interfaces may crash if the
system does not support a thread pointer. at this point, these should
be limited to pthread interfaces, and the number of such cases should
be fewer than before.
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STB_WEAK is only a weak reference for undefined symbols (those with a
section of SHN_UNDEF). otherwise, it's a weak definition. normally
this distinction would not matter, since a relocation referencing a
symbol that also provides a definition (not SHN_UNDEF) will always
succeed in finding the referenced symbol itself. however, in the case
of copy relocations, the referenced symbol itself is ignored in order
to search for another symbol to copy from, and thus it's possible that
no definition is found. in this case, if the symbol being resolved
happened to be a weak definition, it was misinterpreted as a weak
reference, suppressing the error path and causing a crash when the
copy relocation was performed with a null source pointer passed to
memcpy.
there are almost certainly still situations in which invalid
combinations of symbol and relocation types can cause the dynamic
linker to crash (this is pretty much inevitable), but the intent is
that crashes not be possible for symbol/relocation tables produced by
a valid linker.
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this agrees with implementation practice on glibc and BSD systems, and
is the const-correct way to do things; it eliminates warnings from
passing pointers to const. the prototype without const came from
seemingly erroneous man pages.
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this is still experimental and subject to change. for git checkouts,
an attempt is made to record the exact revision to aid in bug reports
and debugging. no version information is recorded in the static libc.a
or binaries it's linked into.
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the affected branch only applies for DSOs that lack standard hash
table and only have the GNU hash table present.
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PAGE_SIZE was hardcoded to 4096, which is historically what most
systems use, but on several archs it is a kernel config parameter,
user space can only know it at execution time from the aux vector.
PAGE_SIZE and PAGESIZE are not defined on archs where page size is
a runtime parameter, applications should use sysconf(_SC_PAGE_SIZE)
to query it. Internally libc code defines PAGE_SIZE to libc.page_size,
which is set to aux[AT_PAGESZ] in __init_libc and early in __dynlink
as well. (Note that libc.page_size can be accessed without GOT, ie.
before relocations are done)
Some fpathconf settings are hardcoded to 4096, these should be actually
queried from the filesystem using statfs.
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if fopen fails for a reason other than ENOENT, we must assume the
intent is that the path file be used. failure may be due to
misconfiguration or intentional resource-exhaustion attack (against
suid programs), in which case falling back to loading libraries from
an unintended path could be dangerous.
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this seems to match what other systems do, and seems useful for
programs that have their libraries and plugins stored relative to the
executable.
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1. an occurrence of ${ORIGIN} before $ORIGIN would be ignored due to
the strstr logic. (note that rpath contains multiple :-delimited paths
to be searched.)
2. data read by readlink was not null-terminated.
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fallback to argv[0] as before. unlike argv[0], AT_EXECFN was a valid
(but possibly relative) pathname for the new program image at the time
the execve syscall was made.
as a special case, ignore AT_EXECFN if it begins with "/proc/", in
order not to give bogus (and possibly harmful) results when fexecve
was used.
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previously, rpath was only honored for direct dependencies. in other
words, if A depends on B and B depends on C, only B's rpath (if any),
not A's rpath, was being searched for C. this limitation made
rpath-based deployment difficult in the presence of multiple levels of
library dependency.
at present, $ORIGIN processing in rpath is still unsupported.
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this commit does not add versioning support; it merely fixes incorrect
lookups of symbols in libraries that contain versioned symbols.
previously, the version information was completely ignored, and
empirically this seems to have resulted in the oldest version being
chosen, but I am uncertain if that behavior was even reliable.
the new behavior being introduced is to completely ignore symbols
which are marked "hidden" (this seems to be the confusing nomenclature
for non-current-version) when versioning is present. this should solve
all problems related to libraries with symbol versioning as long as
all binaries involved are up-to-date (compatible with the
latest-version symbols), and it's the needed behavior for dlsym under
all circumstances.
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this is needed for reused threads in the SIGEV_THREAD timer
notification system, and could be reused elsewhere in the future if
needed, though it should be refactored for such use.
for static linking, __init_tls.c is simply modified to export the TLS
info in a structure with external linkage, rather than using statics.
this perhaps makes the code more clear, since the statics were poorly
named for statics. the new __reset_tls.c is only linked if it is used.
for dynamic linking, the code is in dynlink.c. sharing code with
__copy_tls is not practical since __reset_tls must also re-zero
thread-local bss.
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this is the modern way, and the only way that makes any sense. glibc
has this complicated mechanism with RPATH and RUNPATH that controls
whether RPATH is processed before or after LD_LIBRARY_PATH, presumably
to support legacy binaries, but there is no compelling reason to
support this, and better behavior is obtained by just fixing the
search order.
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this fixes an oversight in the previous commit.
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previously, errno could be meaningless when the caller wrote it to the
dlerror string or stderr. try to make it meaningful. also, fix
incorrect check for over-long program headers and instead actually
support them by allocating memory if needed.
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this can only happen for invalid library files, but they were not
detected reliably because the variable was uninitialized.
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one of the arguments to memcmp may be shorter than the length l-3, and
memcmp is under no obligation not to access past the first byte that
differs. instead use strncmp which conveys the correct semantics. the
performance difference is negligible here and since the code is only
use for shared libc, both functions are already linked anyway.
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