Document struct link_map and interaction with dlinfo (RHEL-47467)
Resolves: RHEL-47467
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Florian Weimer
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glibc-RHEL-47467.patch
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glibc-RHEL-47467.patch
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commit b7d4de086ce7fcc531cdd67a61dc27b5b3eff482
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Author: Florian Weimer <fweimer@redhat.com>
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Date: Mon Aug 5 16:01:12 2024 +0200
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manual: Describe struct link_map, support link maps with dlinfo
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This does not describe how to use RTLD_DI_ORIGIN and l_name
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to reconstruct a full path for the an object. The reason
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is that I think we should not recommend further use of
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RTLD_DI_ORIGIN due to its buffer overflow potential (bug 24298).
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This should be covered by another dlinfo extension. It would
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also obsolete the need for the dladdr approach to obtain
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the file name for the main executable.
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Obtaining the lowest address from load segments in program
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headers is quite clumsy and should be provided directly
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via dlinfo.
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Reviewed-by: Carlos O'Donell <carlos@redhat.com>
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diff --git a/manual/dynlink.texi b/manual/dynlink.texi
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index 39d0a4c4b908e87b..d59dca2b7a916889 100644
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--- a/manual/dynlink.texi
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+++ b/manual/dynlink.texi
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@@ -351,16 +351,119 @@ support the XGETBV instruction.
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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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+@Theglibc{} provides various facilities for querying information from the
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dynamic linker.
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+@deftp {Data Type} {struct link_map}
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+
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+@cindex link map
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+A @dfn{link map} is associated with the main executable and each shared
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+object. Some fields of the link map are accessible to applications and
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+exposed through the @code{struct link_map}. Applications must not modify
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+the link map directly.
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+
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+Pointers to link maps can be obtained from the @code{_r_debug} variable,
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+from the @code{RTLD_DI_LINKMAP} request for @code{dlinfo}, and from the
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+@code{_dl_find_object} function. See below for details.
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+
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+@table @code
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+@item l_addr
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+@cindex load address
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+This field contains the @dfn{load address} of the object. This is the
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+offset that needs to be applied to unrelocated addresses in the object
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+image (as stored on disk) to form an address that can be used at run
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+time for accessing data or running code. For position-dependent
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+executables, the load address is typically zero, and no adjustment is
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+required. For position-independent objects, the @code{l_addr} field
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+usually contains the address of the object's ELF header in the process
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+image. However, this correspondence is not guaranteed because the ELF
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+header might not be mapped at all, and the ELF file as stored on disk
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+might use zero as the lowest virtual address. Due to the second
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+variable, values of the @code{l_addr} field do not necessarily uniquely
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+identify a shared object.
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+
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+On Linux, to obtain the lowest loaded address of the main program, use
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+@code{getauxval} to obtain the @code{AT_PHDR} and @code{AT_PHNUM} values
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+for the current process. Alternatively, call
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+@samp{dlinfo (_r_debug.r_map, &@var{phdr})}
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+to obtain the number of program headers, and the address of the program
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+header array will be stored in @var{phdr}
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+(of type @code{const ElfW(Phdr) *}, as explained below).
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+These values allow processing the array of program headers and the
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+address information in the @code{PT_LOAD} entries among them.
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+This works even when the program was started with an explicit loader
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+invocation.
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+
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+@item l_name
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+For a shared object, this field contains the file name that the
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+@theglibc{} dynamic loader used when opening the object. This can be
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+a relative path (relative to the current directory at process start,
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+or if the object was loaded later, via @code{dlopen} or
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+@code{dlmopen}). Symbolic links are not necessarily resolved.
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+
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+For the main executable, @code{l_name} is @samp{""} (the empty string).
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+(The main executable is not loaded by @theglibc{}, so its file name is
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+not available.) On Linux, the main executable is available as
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+@file{/proc/self/exe} (unless an explicit loader invocation was used to
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+start the program). The file name @file{/proc/self/exe} continues to
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+resolve to the same file even if it is moved within or deleted from the
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+file system. Its current location can be read using @code{readlink}.
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+@xref{Symbolic Links}. (Although @file{/proc/self/exe} is not actually
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+a symbol link, it is only presented as one.) Note that @file{/proc} may
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+not be mounted, in which case @file{/proc/self/exe} is not available.
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+
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+If an explicit loader invocation is used (such as @samp{ld.so
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+/usr/bin/emacs}), the @file{/proc/self/exe} approach does not work
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+because the file name refers to the dynamic linker @code{ld.so}, and not
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+the @code{/usr/bin/emacs} program. An approximation to the executable
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+path is still available in the @code{@var{info}.dli_fname} member after
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+calling @samp{dladdr (_r_debug.r_map->l_ld, &@var{info})}. Note that
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+this could be a relative path, and it is supplied by the process that
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+created the current process, not the kernel, so it could be inaccurate.
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+
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+@item l_ld
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+This is a pointer to the ELF dynamic segment, an array of tag/value
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+pairs that provide various pieces of information that the dynamic
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+linking process uses. On most architectures, addresses in the dynamic
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+segment are relocated at run time, but on some architectures and in some
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+run-time configurations, it is necessary to add the @code{l_addr} field
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+value to obtain a proper address.
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+
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+@item l_prev
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+@itemx l_next
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+These fields are used to maintain a double-linked linked list of all
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+link maps within one @code{dlmopen} namespace. Note that there is
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+currently no thread-safe way to iterate over this list. The
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+callback-based @code{dl_iterate_phdr} interface can be used instead.
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+@end table
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+@end deftp
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+
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+@strong{Portability note:} It is not possible to create a @code{struct
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+link_map} object and pass a pointer to a function that expects a
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+@code{struct link_map *} argument. Only link map pointers initially
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+supplied by @theglibc{} are permitted as arguments. In current versions
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+of @theglibc{}, handles returned by @code{dlopen} and @code{dlmopen} are
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+pointers to link maps. However, this is not a portable assumption, and
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+may even change in future versions of @theglibc{}. To obtain the link
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+map associated with a handle, see @code{dlinfo} and
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+@code{RTLD_DI_LINKMAP} below. If a function accepts both
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+@code{dlopen}/@code{dlmopen} handles and @code{struct link_map} pointers
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+in its @code{void *} argument, that is documented explicitly.
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+
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+@subsection Querying information for loaded objects
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+
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+The @code{dlinfo} function provides access to internal information
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+associated with @code{dlopen}/@code{dlmopen} handles and link maps.
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+
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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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+not have been closed by @code{dlclose}. Alternatively, @var{handle}
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+can be a @code{struct link_map *} value for a link map of an object
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+that has not been closed.
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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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@ -157,7 +157,7 @@ end \
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Summary: The GNU libc libraries
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Name: glibc
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Version: %{glibcversion}
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Release: 143%{?dist}
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Release: 144%{?dist}
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# In general, GPLv2+ is used by programs, LGPLv2+ is used for
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# libraries.
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@ -952,6 +952,7 @@ Patch713: glibc-RHEL-1915-6.patch
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Patch714: glibc-RHEL-1915-7.patch
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Patch715: glibc-RHEL-1915-8.patch
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Patch716: glibc-RHEL-1915-9.patch
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Patch717: glibc-RHEL-47467.patch
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##############################################################################
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# Continued list of core "glibc" package information:
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@ -3111,6 +3112,9 @@ update_gconv_modules_cache ()
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%endif
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%changelog
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* Mon Dec 2 2024 Florian Weimer <fweimer@redhat.com> - 2.34-144
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- Document struct link_map and interaction with dlinfo (RHEL-47467)
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* Wed Nov 20 2024 Patsy Griffin <patsy@redhat.com> - 2.34-143
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- iconv: Support in-place conversions (RHEL-1915)
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