commit 4ff234f6cb erroneously changed
the condition for running certain code at dlopen time to check whether
the library was already relocated rather than whether it already had
its deps[] table filled. this was out of concern over whether the code
under the conditional would be idempotent/safe to call on an
already-loaded libraries. however, I missed a consideration in the
opposite direction: if a library was loaded at program startup rather
than dlopen, its deps[] table was not yet allocated/filled, and
load_deps needs to be called at dlopen time in order for dlsym to be
able to perform dependency-order symbol lookups.
in order to avoid wasteful allocation of lazy-binding relocation
tables for libraries which were already loaded and relocated at
startup, the check for !p->relocated is not deleted entirely, but
moved to apply only to allocation of these dables.
this change was suggested based on testing done by Timo Teräs almost
two years ago; the branch (and probably call prep overhead) in the
inner loop was found to contribute noticably to total symbol lookup
time. this change will make lookup slightly slower if libraries were
built with only the traditional "sysv" ELF hash table, but based on
how much slower lookup tends to be without the gnu hash table, it
seems reasonable to assume that (1) users building without gnu hash
don't care about dynamic linking performance, and (2) the extra time
spent computing the gnu hash is likely to be dominated by the slowness
of the sysv hash table lookup anyway.
such loading is unsafe, and can happen when programs use their own
logic to locate a .so file then pass the absolute pathname to dlopen,
or if an absolute pathname ends up in DT_NEEDED headers. multiple
loads with only the base name were already precluded, provided libc
was named appropriately, by special-casing standard library names.
one function symbol (in the reserved namespace, but public, since it's
part of the crt1 entry point ABI) and one data symbol are checked.
this way we avoid likely false positives, particularly from libraries
interposing and wrapping functions. there is no hard requirement to
avoid breaking such usage, since trying to run a hook before libc is
even initialized is not a supported usage case, but it's friendlier
not to break things.
traditional lazy relocation with call-time plt resolver is
intentionally not implemented, as it is a huge bug surface and demands
significant amounts of arch-specific code and requires ongoing
maintenance to ensure compatibility with applications which make use
of new additions to the arch's register file in passing function
arguments.
some applications, however, depend on the ability to dlopen modules
which have unsatisfied symbol references at the time they are loaded,
either avoiding use of the affected interfaces or manually loading
another module to provide the missing definition via their own module
dependency tracking outside the ELF data structures. while such usage
is non-conforming, failure to support it has been a significant
obstacle for users/distributions trying to support affected software,
particularly the X.org server.
instead of resolving lazy relocations at call time, this patch saves
unresolved GOT/PLT relocations for deferral and retries them after
each subsequent dlopen until they are resolved. since dlopen is the
only time at which the effective global symbol table can change, this
behavior is not observably different from traditional lazy binding,
and the required code is minimal.
when loading libraries with dlopen, the caller can request that the
library's symbols become part of the global symbol table, or that they
only be used for resolving relocations in the loaded library and its
dependencies. in the latter case, a subsequent dlopen of the same
library can upgrade it to global status.
previously, if a library was upgraded from local to global mode, its
symbols entered the symbol lookup search order at the point where the
library was originally loaded. this means that a new call to dlopen
could change the value of a symbol that already had a visible
definition, an inconsistency which applications could observe.
POSIX is unclear whether this should happen or whether it's permitted
to happen, but the resolution of Austin Group issue #982 made it
formally unspecified.
with this patch, a library whose mode is upgraded from local to global
enters the symbol lookup order at the point where it was made global,
so that symbol resolution before and after the upgrade are consistent.
in order to implement this change, the per-dso global flag is replaced
with a separate set of linked-list pointers for participation in the
global symbol table. this permits the order of dso objects for symbol
resolution to differ from the order used for iteration of all loaded
libraries. it also improves performance of find_sym, by avoiding a
branch per iteration and skipping, and especially in the case where
many non-global libraries have been loaded, by allowing the loop to
skip over them entirely. logic for temporarily adding non-global
libraries to the symbol table for relocation purposes is also mildly
simplified.
A weak symbol definition is not special during dynamic linking, so
don't let a strong definition in a later module override it.
(glibc dynamic linker allows overriding weak definitions if
LD_DYNAMIC_WEAK is set, musl does not.)
STB_GNU_UNIQUE means that the symbol is global, even if it is in a
module that's loaded with RTLD_LOCAL, and all references resolve to
the same definition. This semantics is only relevant for c++ plugin
systems and even there it's often not what the user wants (so it can
be turned off in g++ by -fno-gnu-unique when the c++ shared lib is
compiled). In musl just treat it like STB_GLOBAL.
commit d56460c939 introduced this
regression as part of splitting the tls module list out of the dso
list. the new code added to dlopen's failure path to undo the changes
adding the partially-loaded libraries reset the tls_tail pointer
correctly, but did not clear its link to the next list entry. thus, at
least until the next successful dlopen, the list was not terminated
but ended with an invalid next pointer, which __copy_tls attempted to
follow when a new thread was created.
patch by Mikael Vidstedt.
On s390x, the kernel provides AT_SYSINFO_EHDR, but sets it to zero, if the
program being run does not have a program interpreter. This causes
problems when running the dynamic linker directly.
alpha and s390x gratuitously use 64-bit entries (wasting 2x space and
cache utilization) despite the values always being 32-bit.
based on patch by Bobby Bingham, with changes suggested by Alexander
Monakov to use the public Elf_Symndx type from link.h (and make it
properly variable by arch) rather than adding new internal
infrastructure for handling the type.
If a DT_NEEDED entry was the prefix of a reserved library name
(up to the first dot) then it was incorrectly treated as a libc
reserved name.
e.g. libp.so dependency was not loaded as it matched libpthread
reserved name.
this change is made in preparation for adding the mips64 port, which
needs a 64-bit (and mips64-specific) form of the R_INFO macro, but
it's a better abstraction anyway.
based on part of the mips64 port patch by Mahesh Bodapati and Jaydeep
Patil of Imagination Technologies.
commit d56460c939 introduced this bug by
setting up the tls module chain incorrectly when the main app has tls.
the singly-linked list head pointer was setup correctly, but the tail
pointer was not, so the first attempt to append to the list (for a
shared library with tls) would treat the list as empty and effectively
removed the main app from the list. this left all tls module id
numbers off-by-one.
this bug did not appear in any released versions.
this eliminates the last need for the SHARED macro to control how
files in the src tree are compiled. the same code is used for both
libc.a and libc.so, with additional code for the dynamic linker (from
the new ldso tree) being added to libc.so but not libc.a. separate .o
and .lo object files still exist for the src tree, but the only
difference is that the .lo files are built as PIC.
in the future, if/when we add dlopen support for static-linked
programs, much of the code in dynlink.c may be moved back into the src
tree, but properly factored into separate source files. in that case,
the code in the ldso tree will be reduced to just the dynamic linker
entry point, self-relocation, and loading of libraries needed by the
main application.
the function name is still __-prefixed because it requires an asm
wrapper to pass the caller's address in order for RTLD_NEXT to work.
since this was the last function in dynlink.c still used for static
linking, now the whole file is conditional on SHARED being defined.
the ultimate goal of this change is to get all code used in libc.a out
of dynlink.c, so that the dynamic linker code can be moved to its own
tree and object files in the src tree can all be shared between libc.a
and libc.so.
if two or more threads accessed tls in a dso that was loaded after
the threads were created, then __tls_get_new could do out-of-bound
memory access (leading to segfault).
accidentally byte count was used instead of element count when
the new dtv pointer was computed. (dso->new_dtv is (void**).)
it is rare that the same dso provides dtv for several threads,
the crash was not observed in practice, but possible to trigger.
commit ad1cd43a86 eliminated
preprocessor-level omission of references to the init/fini array
symbols from object files going into libc.so. the references are weak,
and the intent was that the linker would resolve them to zero in
libc.so, but instead it leaves undefined references that could be
satisfied at runtime. normally these references would be harmless,
since the code using them does not even get executed, but some older
binutils versions produce a linking error: when linking a program
against libc.so, ld first tries to use the hidden init/fini array
symbols produced by the linker script to satisfy the references in
libc.so, then produces an error because the definitions are hidden.
ideally ld would have already provided definitions of these symbols
when linking libc.so, but the linker script for -shared omits them.
to avoid this situation, the dynamic linker now provides its own dummy
definitions of the init/fini array symbols for libc.so. since they are
hidden, everything binds at ld time and no references remain in the
dynamic symbol table. with modern binutils and --gc-sections, both
the dummy empty array objects and the code referencing them get
dropped at link time, anyway.
the _init and _fini symbols are also switched back to using weak
definitions rather than weak references since the latter behave
somewhat problematically in general, and the weak definition approach
was known to work well.
the nommu kernel shares memory when it can anyway for private
read-only maps, but semantically the map should be private. this can
make a difference when debugging breakpoints are to be used, in which
case the kernel may need to ensure that the mapping is not shared.
the new behavior matches how the kernel FDPIC loader maps the main
program and/or program interpreter (dynamic linker) binary.
this both allows removal of some of the main remaining uses of the
SHARED macro and clears one obstacle to static-linked dlopen support,
which may be added at some point in the future.
specialized single-TLS-module versions of __copy_tls and __reset_tls
are removed and replaced with code adapted from their dynamic-linked
versions, capable of operating on a whole chain of TLS modules, and
use of the dynamic linker's DSO chain (which contains large struct dso
objects) by these functions is replaced with a new chain of struct
tls_module objects containing only the information needed for
implementing TLS. this may also yield some performance benefit
initializing TLS for a new thread when a large number of modules
without TLS have been loaded, since since there is no need to walk
structures for modules without TLS.
use weak definitions that the dynamic linker can override instead of
preprocessor conditionals on SHARED so that the same libc start and
exit code can be used for both static and dynamic linking.
on linux/nommu, non-writable private mappings of files may actually
use memory shared with other processes or the fs cache. the old nommu
loader code (used when mmap with MAP_FIXED fails) simply wrote over
top of the original file mapping, possibly clobbering this shared
memory. no such breakage was observed in practice, but it should have
been possible.
the new code starts by mapping anonymous writable memory on archs that
might support nommu, then maps load segments over top of it, falling
back to read if MAP_FIXED fails. we use an anonymous map rather than a
writable file map to avoid reading more data from disk than needed.
since pages cannot be loaded lazily on fault, in case of large
data/bss, mapping the full file may read a lot of data that will
subsequently be thrown away when processing additional LOAD segments.
as a result, we cannot skip the first LOAD segment when operating in
this mode.
these changes affect only non-FDPIC nommu support.
when a library being loaded has bss (i.e. data segment with
p_memsz>p_filesz), this region needs to be zeroed with a combination
of memset and/or mmap. the regular ELF loader always did this but the
FDPIC code path omitted it, leading to objects in bss having
uninitialized/junk contents.
when determining which module an address belongs to, all function
descriptor ranges must be checked first, in case the allocated memory
falls inside another module's memory range.
dladdr itself must also check addresses against function descriptors
before doing a best-match search against the symbol table. even when
doing the latter (e.g. for code addresses obtained from mcontext_t),
also check whether the best-match was a function, and if so, replace
the result with a function descriptor address. which is the nominal
"base address" of the function and which the caller needs if it
intends to subsequently call the matching function.
since commits 2907afb8db and
6fc30c2493, __dls2 is no longer called
via symbol lookup, but instead uses relative addressing that needs to
be resolved at link time. on some linker versions, and/or if
-Bsymbolic-functions is not used, the linker may leave behind a
dynamic relocation, which is not suitable for bootstrapping the
dynamic linker, if the reference to __dls2 is marked hidden but the
definition is not actually hidden. correcting the definition to use
hidden visibility fixes the problem.
the static-PIE entry point rcrt1 was likewise affected and is also
fixed by this patch.
lookup the dso an address falls in based on the loadmap and not just a
base/length. fix the main app's fake loadmap used when loaded by a
non-fdpic-aware loader so that it does not cover the whole memory
space.
function descriptor addresses are also matched for future use by
dladdr, but reverse lookups of function descriptors via dladdr have
not been implemented yet. some revisions may be needed in the future
once reclaim_gaps supports fdpic, so that function descriptors
allocated in reclaimed heap space do not get detected as belonging to
the module whose gaps they were allocated in.
previously these resolved to the code address rather than the address
of the function descriptor.
the conditions for accepting or rejecting symbols are quite
inconsistent between the different points in the dynamic linker code
where such decisions are made. this commit attempts to be at least as
correct as anything already there, but does not improve consistency.
it has been tested to correctly avoid symbols that are merely
references to functions defined in other modules, at least in simple
usage, but at some point all symbol lookup logic should be reviewed
and refactored/unified.
this change is needed to be compatible with fdpic, where some of the
main application's relocations may be performed as part of the crt1
entry point. if we call init functions before passing control, these
relocations will not yet have been performed, and the init code will
potentially make use of invalid pointers.
conceptually, no code provided by the application or third-party
libraries should run before the application entry point. the
difference is not observable to programs using the crt1 we provide,
but it could come into play if custom entry point code is used, so
it's better to be doing this right anyway.
previously, the normal ELF library loading code was used even for
fdpic, so only the kernel-loaded dynamic linker and main app could
benefit from separate placement of segments and shared text.
at this point not all functionality is complete. the dynamic linker
itself, and main app if it is also loaded by the kernel, take
advantage of fdpic and do not need constant displacement between
segments, but additional libraries loaded by the dynamic linker follow
normal ELF semantics for mapping still. this fully works, but does not
admit shared text on nommu.
in terms of actual functional correctness, dlsym's results are
presently incorrect for function symbols, RTLD_NEXT fails to identify
the caller correctly, and dladdr fails almost entirely.
with the dynamic linker entry point working, support for static pie is
automatically included, but linking the main application as ET_DYN
(pie) probably does not make sense for fdpic anyway. ET_EXEC is
equally relocatable but more efficient at representing relocations.
the fdpic code will need to count symbols, and it may be useful
elsewhere in the future too. counting is trivial as long as sysv hash
is present, but for gnu-hash-only libraries it's complex.
the behavior of the count is changed slightly: we now include symbols
that are not accessible by the gnu hash table in the count. this may
make dladdr slightly slower. if this is a problem, dladdr can subtract
out the part that should not be accessible. unlike in the old code,
subtracting this out is easy even in the fast path where sysv hash is
available too.
these are in do_relocs. the first one was omitted in commit
301335a80b because it slightly changes
code (using dso->base rather than cached local var base) and would
have prevented easy verification. the other was an oversight.
for ordinary ELF with fixed segment displacements, load address
computation is simply adding the base load address. but for FDPIC,
each segment has its own load address, and virtual addresses need to
be adjusted according to the segment they fall in. abstracting this
computation is the first step to making the dynamic linker ready for
FDPIC.
for this first commit, a macro is used rather than a function in order
to facilitate correctness checking. I have verified that the generated
code does not change on my i386 build.
The callers need to check the value of the pointer anyway, so make
them pass the pointer to gnu_lookup instead of reloading it there.
Reorder gnu_lookup arguments so that always-used ones are listed
first. GCC can choose a calling convention with arguments in registers
(e.g. up to 3 arguments in eax, ecx, edx on x86), but cannot reorder
the arguments for static functions.
Rich Felker
Szabolcs Nagy
rofl0r
Bobby Bingham
Felix Fietkau
Alexander Monakov