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91c2e6c3db
The information is theoretically available via dl_iterate_phdr as
well, but that approach is very slow if there are many shared
objects.
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@rehdat.com>
(cherry picked from commit d056c21213)
110 lines
4.2 KiB
Plaintext
110 lines
4.2 KiB
Plaintext
@node Dynamic Linker
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@c @node Dynamic Linker, Internal Probes, Threads, Top
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@c %MENU% Loading programs and shared objects.
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@chapter Dynamic Linker
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@cindex dynamic linker
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@cindex dynamic loader
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The @dfn{dynamic linker} is responsible for loading dynamically linked
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programs and their dependencies (in the form of shared objects). The
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dynamic linker in @theglibc{} also supports loading shared objects (such
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as plugins) later at run time.
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Dynamic linkers are sometimes called @dfn{dynamic loaders}.
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@menu
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* Dynamic Linker Introspection:: Interfaces for querying mapping information.
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@end menu
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@node Dynamic Linker Introspection
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@section Dynamic Linker Introspection
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@Theglibc{} provides various functions for querying information from the
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dynamic linker.
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@deftypefun {int} dlinfo (void *@var{handle}, int @var{request}, void *@var{arg})
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@safety{@mtsafe{}@asunsafe{@asucorrupt{}}@acunsafe{@acucorrupt{}}}
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@standards{GNU, dlfcn.h}
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This function returns information about @var{handle} in the memory
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location @var{arg}, based on @var{request}. The @var{handle} argument
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must be a pointer returned by @code{dlopen} or @code{dlmopen}; it must
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not have been closed by @code{dlclose}.
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On success, @code{dlinfo} returns 0 for most request types; exceptions
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are noted below. If there is an error, the function returns @math{-1},
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and @code{dlerror} can be used to obtain a corresponding error message.
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The following operations are defined for use with @var{request}:
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@vtable @code
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@item RTLD_DI_LINKMAP
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The corresponding @code{struct link_map} pointer for @var{handle} is
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written to @code{*@var{arg}}. The @var{arg} argument must be the
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address of an object of type @code{struct link_map *}.
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@item RTLD_DI_LMID
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The namespace identifier of @var{handle} is written to
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@code{*@var{arg}}. The @var{arg} argument must be the address of an
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object of type @code{Lmid_t}.
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@item RTLD_DI_ORIGIN
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The value of the @code{$ORIGIN} dynamic string token for @var{handle} is
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written to the character array starting at @var{arg} as a
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null-terminated string.
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This request type should not be used because it is prone to buffer
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overflows.
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@item RTLD_DI_SERINFO
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@itemx RTLD_DI_SERINFOSIZE
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These requests can be used to obtain search path information for
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@var{handle}. For both requests, @var{arg} must point to a
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@code{Dl_serinfo} object. The @code{RTLD_DI_SERINFOSIZE} request must
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be made first; it updates the @code{dls_size} and @code{dls_cnt} members
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of the @code{Dl_serinfo} object. The caller should then allocate memory
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to store at least @code{dls_size} bytes and pass that buffer to a
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@code{RTLD_DI_SERINFO} request. This second request fills the
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@code{dls_serpath} array. The number of array elements was returned in
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the @code{dls_cnt} member in the initial @code{RTLD_DI_SERINFOSIZE}
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request. The caller is responsible for freeing the allocated buffer.
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This interface is prone to buffer overflows in multi-threaded processes
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because the required size can change between the
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@code{RTLD_DI_SERINFOSIZE} and @code{RTLD_DI_SERINFO} requests.
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@item RTLD_DI_TLS_DATA
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This request writes the address of the TLS block (in the current thread)
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for the shared object identified by @var{handle} to @code{*@var{arg}}.
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The argument @var{arg} must be the address of an object of type
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@code{void *}. A null pointer is written if the object does not have
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any associated TLS block.
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@item RTLD_DI_TLS_MODID
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This request writes the TLS module ID for the shared object @var{handle}
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to @code{*@var{arg}}. The argument @var{arg} must be the address of an
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object of type @code{size_t}. The module ID is zero if the object
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does not have an associated TLS block.
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@item RTLD_DI_PHDR
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This request writes the address of the program header array to
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@code{*@var{arg}}. The argument @var{arg} must be the address of an
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object of type @code{const ElfW(Phdr) *} (that is,
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@code{const Elf32_Phdr *} or @code{const Elf64_Phdr *}, as appropriate
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for the current architecture). For this request, the value returned by
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@code{dlinfo} is the number of program headers in the program header
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array.
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@end vtable
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The @code{dlinfo} function is a GNU extension.
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@end deftypefun
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@c FIXME these are undocumented:
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@c dladdr
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@c dladdr1
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@c dlclose
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@c dlerror
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@c dlmopen
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@c dlopen
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@c dlsym
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@c dlvsym
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