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[MTASCsft PATCH 08/??] MT-, AS- and AC-Safety docs: manual/debug.texi
- From: Alexandre Oliva <aoliva at redhat dot com>
- To: codonell at redhat dot com
- Cc: libc-alpha at sourceware dot org
- Date: Thu, 23 Jan 2014 17:23:23 -0200
- Subject: [MTASCsft PATCH 08/??] MT-, AS- and AC-Safety docs: manual/debug.texi
- Authentication-results: sourceware.org; auth=none
- References: <ortxelb5zd dot fsf at livre dot home> <or4n4uoncj dot fsf at livre dot home>
There's some uncertainty here WRT the uses of dladdr, because the
dynamic loader hasn't been fully reviewed, but I reviewed the dladdr
code paths enough to be somewhat confident with the assessment that
follows.
Thanks to Siddhesh for pointing out (long ago) that backtrace had
platform-specific implementations. I'd missed that in my first review.
for ChangeLog
* manual/debug.texi: Document MTASC-safety properties.
---
manual/debug.texi | 26 ++++++++++++++++++++++++++
1 file changed, 26 insertions(+)
diff --git a/manual/debug.texi b/manual/debug.texi
index 1db9c18..25492c3 100644
--- a/manual/debug.texi
+++ b/manual/debug.texi
@@ -36,6 +36,16 @@ and manipulate backtraces of the current thread.
@comment execinfo.h
@comment GNU
@deftypefun int backtrace (void **@var{buffer}, int @var{size})
+@safety{@prelim{}@mtsafe{}@asunsafe{@asuinit{} @ascuheap{} @ascudlopen{} @ascuplugin{} @asulock{}}@acunsafe{@acuinit{} @acsmem{} @aculock{} @acsfd{}}}
+@c The generic implementation just does pointer chasing within the local
+@c stack, without any guarantees that this will handle signal frames
+@c correctly, so it's AS-Unsafe to begin with. However, most (all?)
+@c arches defer to libgcc_s's _Unwind_* implementation, dlopening
+@c libgcc_s.so to that end except in a static version of libc.
+@c libgcc_s's implementation may in turn defer to libunwind. We can't
+@c assume those implementations are AS- or AC-safe, but even if we
+@c could, our own initialization path isn't, and libgcc's implementation
+@c calls malloc and performs internal locking, so...
The @code{backtrace} function obtains a backtrace for the current
thread, as a list of pointers, and places the information into
@var{buffer}. The argument @var{size} should be the number of
@@ -56,6 +66,17 @@ interpreting the stack contents correctly.
@comment execinfo.h
@comment GNU
@deftypefun {char **} backtrace_symbols (void *const *@var{buffer}, int @var{size})
+@safety{@prelim{}@mtsafe{}@asunsafe{@ascuheap{}}@acunsafe{@acsmem{} @aculock{}}}
+@c Collects info returned by _dl_addr in an auto array, allocates memory
+@c for the whole return buffer with malloc then sprintfs into it storing
+@c pointers to the strings into the array entries in the buffer.
+@c _dl_addr takes the recursive dl_load_lock then calls
+@c _dl_find_dso_for_object and determine_info.
+@c _dl_find_dso_for_object calls _dl-addr_inside_object.
+@c All of them are safe as long as the lock is held.
+@c @asucorrupt? It doesn't look like the dynamic loader's data
+@c structures could be in an inconsistent state that would cause
+@c malfunction here.
The @code{backtrace_symbols} function translates the information
obtained from the @code{backtrace} function into an array of strings.
The argument @var{buffer} should be a pointer to an array of addresses
@@ -88,6 +109,11 @@ cannot be obtained.
@comment execinfo.h
@comment GNU
@deftypefun void backtrace_symbols_fd (void *const *@var{buffer}, int @var{size}, int @var{fd})
+@safety{@prelim{}@mtsafe{}@assafe{}@acunsafe{@aculock{}}}
+@c Single loop of _dl_addr over addresses, collecting info into an iovec
+@c written out with a writev call per iteration. Addresses and offsets
+@c are converted to hex in auto buffers, so the only potential issue
+@c here is leaking the dl lock in case of cancellation.
The @code{backtrace_symbols_fd} function performs the same translation
as the function @code{backtrace_symbols} function. Instead of returning
the strings to the caller, it writes the strings to the file descriptor
--
Alexandre Oliva, freedom fighter http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/ FSF Latin America board member
Free Software Evangelist Red Hat Brazil Toolchain Engineer