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the return address was being truncated to 32 bits, preventing the
dlsym code from determining which module contains the calling code.
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this does not change behavior, but the idea is to avoid letting other
code build up between these two points, whereby the environment
variables might get used before security it checked.
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the asm wrapper is needed to get the return address without
compiler-specific extensions.
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instead of creating temp dso objects on the stack and moving them to
the heap if dlopen/dlsym are used, use static objects to begin with,
and just donate them to malloc if we no longer need them.
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this is mostly useless for shared libs (though it could help for
prelink-like purposes); the intended use case is for adding support
for calling the dynamic linker directly to run a program, as in:
./libc.so ./a.out foo
this usage is not yet supported.
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prior to this change, copy relocations for initialized pointer
variables would not reflect the relocated contents of the pointer.
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deps can be null if a library has no dependencies (such as libc itself)
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basically we temporarily make the library and all its dependencies
part of the global namespace but only for the duration of performing
relocations, then return them to their former state.
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some of the code is not yet used, and is in preparation for dlopen
which needs to be able to handle failure loading libraries without
terminating the program.
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1. search was wrongly beginning with lib itself rather than dso head
2. inconsistent resolution of function pointers for functions in plt
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first, use $LD_LIBRARY_PATH unless suid. if that fails, read path from
/etc/ld-musl-$ARCH.path and fallback to a builtin default.
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eventually (once dlopen exists) this behavior will be conditional on
dlopen/dlsym not being reachable.
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the use of this test will be much stricter than glibc and other
typical implementations; the environment will not be honored
whatsoever unless the program is confirmed non-suid/sgid by the aux
vector the kernel passed in. no fallback to slow syscall-based
checking is used if the kernel fails to provide the information; we
simply assume the worst (suid) in this case and refuse to honor
environment.
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leaving it uninitialized caused unpredictable crashes or worse due to
calling an indeterminate function pointer.
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some notes:
- library search path is hard coded
- x86_64 code is untested and may not work
- dlopen/dlsym is not yet implemented
- relocations in read-only memory won't work
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prefer using visibility=hidden for __libc internal data, rather than
an accessor function, if the compiler has visibility.
optimize with -O3 for PIC targets (shared library). without heavy
inlining, reloading the GOT register in small functions kills
performance. 20-30% size increase for a single libc.so is not a big
deal, compared to comparaible size increase in every static binaries.
use -Bsymbolic-functions, not -Bsymbolic. global variables are subject
to COPY relocations, and thus binding their addresses in the library
at link time will cause library functions to read the wrong (original)
copies instead of the copies made in the main program's bss section.
add entry point, _start, for dynamic linker.
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