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Diffstat (limited to 'elf/rtld-debugger-interface.txt')
| -rw-r--r-- | elf/rtld-debugger-interface.txt | 122 |
1 files changed, 122 insertions, 0 deletions
diff --git a/elf/rtld-debugger-interface.txt b/elf/rtld-debugger-interface.txt new file mode 100644 index 0000000000..61bc99e4b0 --- /dev/null +++ b/elf/rtld-debugger-interface.txt @@ -0,0 +1,122 @@ +Standard debugger interface +=========================== + +The run-time linker exposes a rendezvous structure to allow debuggers +to interface with it. This structure, r_debug, is defined in link.h. +If the executable's dynamic section has a DT_DEBUG element, the +run-time linker sets that element's value to the address where this +structure can be found. + +The r_debug structure contains (amongst others) the following fields: + + struct link_map *r_map: + A linked list of loaded objects. + + enum { RT_CONSISTENT, RT_ADD, RT_DELETE } r_state: + The current state of the r_map list. RT_CONSISTENT means that r_map + is not currently being modified and may safely be inspected. RT_ADD + means that an object is being added to r_map, and that the list is + not guaranteed to be consistent. Likewise RT_DELETE means that an + object is being removed from the list. + + ElfW(Addr) r_brk: + The address of a function internal to the run-time linker which is + called whenever r_state is changed. The debugger should set a + breakpoint at this address if it wants to notice mapping changes. + +This protocol is widely supported, but somewhat limited in that it +has no provision to provide access to multiple namespaces, and that +the notifications (via r_brk) only refer to changes to r_map--the +debugger is notified that a new object has been added, for instance, +but there is no way for the debugger to discover whether any of the +objects in the link-map have been relocated or not. + + +Probe-based debugger interface +============================== + +Systemtap is a dynamic tracing/instrumenting tool available on Linux. +Probes that are not fired at run time have close to zero overhead. +glibc contains a number of probes that debuggers can set breakpoints +on in order to notice certain events. + +All rtld probes have the following arguments: + + arg1: Lmid_t lmid: + The link-map ID of the link-map list that the object was loaded + into. This will be LM_ID_BASE for the application's main link-map + list, or some other value for different namespaces. + + arg2: struct r_debug *r_debug: + A pointer to the r_debug structure containing the link-map list + that the object was loaded into. This will be the value stored in + DT_DEBUG for the application's main link-map list, or some other + value for different namespaces. + +map_complete and reloc_complete may have the following additional +argument: + + arg3: struct link_map *new: + A pointer which, if not NULL, points to the entry in the specified + r_debug structure's link-map list corresponding to the first new + object to have been mapped or relocated, with new->l_next pointing + to the link-map of the next new object to have been mapped or + relocated, and so on. Note that because `new' is an entry in a + larger list, new->l_prev (if not NULL) will point to what was the + last link-map in the link-map list prior to the new objects being + mapped or relocated. + +The following probes are available: + + init_start: + This is called once, when the linker is about to fill in the main + r_debug structure at application startup. init_start always has + lmid set to LM_ID_BASE and r_debug set to the value stored in + DT_DEBUG. r_debug is not guaranteed to be consistent until + init_complete is fired. + + init_complete: + This is called once, when the linker has filled in the main + r_debug structure at application startup. init_complete always + has lmid set to LM_ID_BASE and r_debug set to the value stored + in DT_DEBUG. The r_debug structure is consistent and may be + inspected, and all objects in the link-map are guaranteed to + have been relocated. + + map_start: + The linker is about to map new objects into the specified + namespace. The namespace's r_debug structure is not guaranteed + to be consistent until a corresponding map_complete is fired. + + map_complete: + The linker has finished mapping new objects into the specified + namespace. The namespace's r_debug structure is consistent and + may be inspected, although objects in the namespace's link-map + are not guaranteed to have been relocated. + + map_failed: + The linker failed while attempting to map new objects into + the specified namespace. The namespace's r_debug structure + is consistent and may be inspected. + + reloc_start: + The linker is about to relocate all unrelocated objects in the + specified namespace. The namespace's r_debug structure is not + guaranteed to be consistent until a corresponding reloc_complete + is fired. + + reloc_complete: + The linker has relocated all objects in the specified namespace. + The namespace's r_debug structure is consistent and may be + inspected, and all objects in the namespace's link-map are + guaranteed to have been relocated. + + unmap_start: + The linker is about to remove objects from the specified + namespace. The namespace's r_debug structure is not guaranteed to + be consistent until a corresponding unmap_complete is fired. + + unmap_complete: + The linker has finished removing objects into the specified + namespace. The namespace's r_debug structure is consistent and + may be inspected. |