From 31be941e4367c001b2009308839db5c67bf9dcbc Mon Sep 17 00:00:00 2001 From: Simon Kissane Date: Sat, 11 Feb 2023 20:12:13 +1100 Subject: [PATCH] gmon: improve mcount overflow handling [BZ# 27576] When mcount overflows, no gmon.out file is generated, but no message is printed to the user, leaving the user with no idea why, and thinking maybe there is some bug - which is how BZ 27576 ended up being logged. Print a message to stderr in this case so the user knows what is going on. As a comment in sys/gmon.h acknowledges, the hardcoded MAXARCS value is too small for some large applications, including the test case in that BZ. Rather than increase it, add tunables to enable MINARCS and MAXARCS to be overridden at runtime (glibc.gmon.minarcs and glibc.gmon.maxarcs). So if a user gets the mcount overflow error, they can try increasing maxarcs (they might need to increase minarcs too if the heuristic is wrong in their case.) Note setting minarcs/maxarcs too large can cause monstartup to fail with an out of memory error. If you set them large enough, it can cause an integer overflow in calculating the buffer size. I haven't done anything to defend against that - it would not generally be a security vulnerability, since these tunables will be ignored in suid/sgid programs (due to the SXID_ERASE default), and if you can set GLIBC_TUNABLES in the environment of a process, you can take it over anyway (LD_PRELOAD, LD_LIBRARY_PATH, etc). I thought about modifying the code of monstartup to defend against integer overflows, but doing so is complicated, and I realise the existing code is susceptible to them even prior to this change (e.g. try passing a pathologically large highpc argument to monstartup), so I decided just to leave that possibility in-place. Add a test case which demonstrates mcount overflow and the tunables. Document the new tunables in the manual. Signed-off-by: Simon Kissane Reviewed-by: DJ Delorie --- elf/dl-tunables.list | 13 ++++++ gmon/Makefile | 22 +++++++++- gmon/gmon.c | 29 +++++++++++-- gmon/mcount.c | 5 +++ gmon/sys/gmon.h | 6 ++- gmon/tst-mcount-overflow-check.sh | 45 +++++++++++++++++++ gmon/tst-mcount-overflow.c | 72 +++++++++++++++++++++++++++++++ manual/tunables.texi | 59 +++++++++++++++++++++++++ 8 files changed, 244 insertions(+), 7 deletions(-) create mode 100644 gmon/tst-mcount-overflow-check.sh create mode 100644 gmon/tst-mcount-overflow.c diff --git a/elf/dl-tunables.list b/elf/dl-tunables.list index a685f5cdbc..695ba7192e 100644 --- a/elf/dl-tunables.list +++ b/elf/dl-tunables.list @@ -170,4 +170,17 @@ glibc { default: 2 } } + + gmon { + minarcs { + type: INT_32 + minval: 50 + default: 50 + } + maxarcs { + type: INT_32 + minval: 50 + default: 1048576 + } + } } diff --git a/gmon/Makefile b/gmon/Makefile index 4dd5adb80b..83837dd689 100644 --- a/gmon/Makefile +++ b/gmon/Makefile @@ -25,7 +25,7 @@ include ../Makeconfig headers := sys/gmon.h sys/gmon_out.h sys/profil.h routines := gmon mcount profil sprofil prof-freq -tests = tst-sprofil tst-gmon +tests = tst-sprofil tst-gmon tst-mcount-overflow ifeq ($(build-profile),yes) tests += tst-profile-static tests-static += tst-profile-static @@ -56,6 +56,18 @@ ifeq ($(run-built-tests),yes) tests-special += $(objpfx)tst-gmon-gprof.out endif +CFLAGS-tst-mcount-overflow.c := -fno-omit-frame-pointer -pg +tst-mcount-overflow-no-pie = yes +CRT-tst-mcount-overflow := $(csu-objpfx)g$(start-installed-name) +# Intentionally use invalid config where maxarcs&1 1>/dev/null | cat +ifeq ($(run-built-tests),yes) +tests-special += $(objpfx)tst-mcount-overflow-check.out +endif + CFLAGS-tst-gmon-static.c := $(PIE-ccflag) -fno-omit-frame-pointer -pg CRT-tst-gmon-static := $(csu-objpfx)g$(static-start-installed-name) tst-gmon-static-no-pie = yes @@ -103,6 +115,14 @@ $(objpfx)tst-gmon.out: clean-tst-gmon-data clean-tst-gmon-data: rm -f $(objpfx)tst-gmon.data.* +$(objpfx)tst-mcount-overflow.o: clean-tst-mcount-overflow-data +clean-tst-mcount-overflow-data: + rm -f $(objpfx)tst-mcount-overflow.data.* + +$(objpfx)tst-mcount-overflow-check.out: tst-mcount-overflow-check.sh $(objpfx)tst-mcount-overflow.out + $(SHELL) $< $(objpfx)tst-mcount-overflow > $@; \ + $(evaluate-test) + $(objpfx)tst-gmon-gprof.out: tst-gmon-gprof.sh $(objpfx)tst-gmon.out $(SHELL) $< $(GPROF) $(objpfx)tst-gmon $(objpfx)tst-gmon.data.* > $@; \ $(evaluate-test) diff --git a/gmon/gmon.c b/gmon/gmon.c index bf76358d5b..689bf80141 100644 --- a/gmon/gmon.c +++ b/gmon/gmon.c @@ -46,6 +46,11 @@ #include #include +#if HAVE_TUNABLES +# define TUNABLE_NAMESPACE gmon +# include +#endif + #ifdef PIC # include @@ -124,6 +129,22 @@ __monstartup (u_long lowpc, u_long highpc) int o; char *cp; struct gmonparam *p = &_gmonparam; + long int minarcs, maxarcs; + +#if HAVE_TUNABLES + /* Read minarcs/maxarcs tunables. */ + minarcs = TUNABLE_GET (minarcs, int32_t, NULL); + maxarcs = TUNABLE_GET (maxarcs, int32_t, NULL); + if (maxarcs < minarcs) + { + ERR("monstartup: maxarcs < minarcs, setting maxarcs = minarcs\n"); + maxarcs = minarcs; + } +#else + /* No tunables, we use hardcoded defaults */ + minarcs = MINARCS; + maxarcs = MAXARCS; +#endif /* * round lowpc and highpc to multiples of the density we're using @@ -146,10 +167,10 @@ __monstartup (u_long lowpc, u_long highpc) } p->fromssize = ROUNDUP(p->textsize / HASHFRACTION, sizeof(*p->froms)); p->tolimit = p->textsize * ARCDENSITY / 100; - if (p->tolimit < MINARCS) - p->tolimit = MINARCS; - else if (p->tolimit > MAXARCS) - p->tolimit = MAXARCS; + if (p->tolimit < minarcs) + p->tolimit = minarcs; + else if (p->tolimit > maxarcs) + p->tolimit = maxarcs; p->tossize = p->tolimit * sizeof(struct tostruct); cp = calloc (p->kcountsize + p->fromssize + p->tossize, 1); diff --git a/gmon/mcount.c b/gmon/mcount.c index 9d4a1a50fa..f7180fdb83 100644 --- a/gmon/mcount.c +++ b/gmon/mcount.c @@ -41,6 +41,10 @@ static char sccsid[] = "@(#)mcount.c 8.1 (Berkeley) 6/4/93"; #include +#include +#include +#define ERR(s) __write_nocancel (STDERR_FILENO, s, sizeof (s) - 1) + /* * mcount is called on entry to each function compiled with the profiling * switch set. _mcount(), which is declared in a machine-dependent way @@ -170,6 +174,7 @@ done: return; overflow: p->state = GMON_PROF_ERROR; + ERR("mcount: call graph buffer size limit exceeded, gmon.out will not be generated\n"); return; } diff --git a/gmon/sys/gmon.h b/gmon/sys/gmon.h index b4cc3b043a..af0582a371 100644 --- a/gmon/sys/gmon.h +++ b/gmon/sys/gmon.h @@ -111,6 +111,8 @@ extern struct __bb *__bb_head; * Always allocate at least this many tostructs. This * hides the inadequacy of the ARCDENSITY heuristic, at least * for small programs. + * + * Value can be overridden at runtime by glibc.gmon.minarcs tunable. */ #define MINARCS 50 @@ -124,8 +126,8 @@ extern struct __bb *__bb_head; * Used to be max representable value of ARCINDEX minus 2, but now * that ARCINDEX is a long, that's too large; we don't really want * to allow a 48 gigabyte table. - * The old value of 1<<16 wasn't high enough in practice for large C++ - * programs; will 1<<20 be adequate for long? FIXME + * + * Value can be overridden at runtime by glibc.gmon.maxarcs tunable. */ #define MAXARCS (1 << 20) diff --git a/gmon/tst-mcount-overflow-check.sh b/gmon/tst-mcount-overflow-check.sh new file mode 100644 index 0000000000..27eb5538fd --- /dev/null +++ b/gmon/tst-mcount-overflow-check.sh @@ -0,0 +1,45 @@ +#!/bin/sh +# Test expected messages generated when mcount overflows +# Copyright (C) 2017-2023 Free Software Foundation, Inc. +# Copyright The GNU Toolchain Authors. +# This file is part of the GNU C Library. + +# The GNU C Library is free software; you can redistribute it and/or +# modify it under the terms of the GNU Lesser General Public +# License as published by the Free Software Foundation; either +# version 2.1 of the License, or (at your option) any later version. + +# The GNU C Library is distributed in the hope that it will be useful, +# but WITHOUT ANY WARRANTY; without even the implied warranty of +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU +# Lesser General Public License for more details. + +# You should have received a copy of the GNU Lesser General Public +# License along with the GNU C Library; if not, see +# . + +LC_ALL=C +export LC_ALL +set -e +exec 2>&1 + +program="$1" + +check_msg() { + if ! grep -q "$1" "$program.out"; then + echo "FAIL: expected message not in output: $1" + exit 1 + fi +} + +check_msg 'monstartup: maxarcs < minarcs, setting maxarcs = minarcs' +check_msg 'mcount: call graph buffer size limit exceeded, gmon.out will not be generated' + +for data_file in $1.data.*; do + if [ -f "$data_file" ]; then + echo "FAIL: expected no data files, but found $data_file" + exit 1 + fi +done + +echo PASS diff --git a/gmon/tst-mcount-overflow.c b/gmon/tst-mcount-overflow.c new file mode 100644 index 0000000000..06cc93ef87 --- /dev/null +++ b/gmon/tst-mcount-overflow.c @@ -0,0 +1,72 @@ +/* Test program to trigger mcount overflow in profiling collection. + Copyright (C) 2017-2023 Free Software Foundation, Inc. + This file is part of the GNU C Library. + + The GNU C Library is free software; you can redistribute it and/or + modify it under the terms of the GNU Lesser General Public + License as published by the Free Software Foundation; either + version 2.1 of the License, or (at your option) any later version. + + The GNU C Library is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU + Lesser General Public License for more details. + + You should have received a copy of the GNU Lesser General Public + License along with the GNU C Library; if not, see + . */ + +/* Program with sufficiently complex, yet pointless, call graph + that it will trigger an mcount overflow, when you set the + minarcs/maxarcs tunables to very low values. */ + +#define PREVENT_TAIL_CALL asm volatile ("") + +/* Calls REP(n) macro 16 times, for n=0..15. + * You need to define REP(n) before using this. + */ +#define REPS \ + REP(0) REP(1) REP(2) REP(3) REP(4) REP(5) REP(6) REP(7) \ + REP(8) REP(9) REP(10) REP(11) REP(12) REP(13) REP(14) REP(15) + +/* Defines 16 leaf functions named f1_0 to f1_15 */ +#define REP(n) \ + __attribute__ ((noinline, noclone, weak)) void f1_##n (void) {}; +REPS +#undef REP + +/* Calls all 16 leaf functions f1_* in succession */ +__attribute__ ((noinline, noclone, weak)) void +f2 (void) +{ +# define REP(n) f1_##n(); + REPS +# undef REP + PREVENT_TAIL_CALL; +} + +/* Defines 16 functions named f2_0 to f2_15, which all just call f2 */ +#define REP(n) \ + __attribute__ ((noinline, noclone, weak)) void \ + f2_##n (void) { f2(); PREVENT_TAIL_CALL; }; +REPS +#undef REP + +__attribute__ ((noinline, noclone, weak)) void +f3 (int count) +{ + for (int i = 0; i < count; ++i) + { + /* Calls f1_0(), f2_0(), f1_1(), f2_1(), f3_0(), etc */ +# define REP(n) f1_##n(); f2_##n(); + REPS +# undef REP + } +} + +int +main (void) +{ + f3 (1000); + return 0; +} diff --git a/manual/tunables.texi b/manual/tunables.texi index c2630b83ab..0be7231e36 100644 --- a/manual/tunables.texi +++ b/manual/tunables.texi @@ -77,6 +77,9 @@ glibc.malloc.check: 0 (min: 0, max: 3) capabilities seen by @theglibc{} * Memory Related Tunables:: Tunables that control the use of memory by @theglibc{}. +* gmon Tunables:: Tunables that control the gmon profiler, used in + conjunction with gprof + @end menu @node Tunable names @@ -616,3 +619,59 @@ support in the kernel if this tunable has any non-zero value. The default value is @samp{0}, which disables all memory tagging. @end deftp + +@node gmon Tunables +@section gmon Tunables +@cindex gmon tunables + +@deftp {Tunable namespace} glibc.gmon +This tunable namespace affects the behaviour of the gmon profiler. +gmon is a component of @theglibc{} which is normally used in +conjunction with gprof. + +When GCC compiles a program with the @code{-pg} option, it instruments +the program with calls to the @code{mcount} function, to record the +program's call graph. At program startup, a memory buffer is allocated +to store this call graph; the size of the buffer is calculated using a +heuristic based on code size. If during execution, the buffer is found +to be too small, profiling will be aborted and no @file{gmon.out} file +will be produced. In that case, you will see the following message +printed to standard error: + +@example +mcount: call graph buffer size limit exceeded, gmon.out will not be generated +@end example + +Most of the symbols discussed in this section are defined in the header +@code{sys/gmon.h}. However, some symbols (for example @code{mcount}) +are not defined in any header file, since they are only intended to be +called from code generated by the compiler. +@end deftp + +@deftp Tunable glibc.mem.minarcs +The heuristic for sizing the call graph buffer is known to be +insufficient for small programs; hence, the calculated value is clamped +to be at least a minimum size. The default minimum (in units of +call graph entries, @code{struct tostruct}), is given by the macro +@code{MINARCS}. If you have some program with an unusually complex +call graph, for which the heuristic fails to allocate enough space, +you can use this tunable to increase the minimum to a larger value. +@end deftp + +@deftp Tunable glibc.mem.maxarcs +To prevent excessive memory consumption when profiling very large +programs, the call graph buffer is allowed to have a maximum of +@code{MAXARCS} entries. For some very large programs, the default +value of @code{MAXARCS} defined in @file{sys/gmon.h} is too small; in +that case, you can use this tunable to increase it. + +Note the value of the @code{maxarcs} tunable must be greater or equal +to that of the @code{minarcs} tunable; if this constraint is violated, +a warning will printed to standard error at program startup, and +the @code{minarcs} value will be used as the maximum as well. + +Setting either tunable too high may result in a call graph buffer +whose size exceeds the available memory; in that case, an out of memory +error will be printed at program startup, the profiler will be +disabled, and no @file{gmon.out} file will be generated. +@end deftp