This patch adds the required @order directives to preserve the
GLIBC_PRIVATE ABI.

commit 31be941e4367c001b2009308839db5c67bf9dcbc
Author: Simon Kissane <skissane@gmail.com>
Date:   Sat Feb 11 20:12:13 2023 +1100

    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 <skissane@gmail.com>
    Reviewed-by: DJ Delorie <dj@redhat.com>

Conflicts:
	manual/tunables.texi
	  (missing tunables downstream)

diff --git a/elf/dl-tunables.list b/elf/dl-tunables.list
index f11ca5b3e8b09b43..dc2999796042dbaf 100644
--- a/elf/dl-tunables.list
+++ b/elf/dl-tunables.list
@@ -149,4 +149,17 @@ 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 d94593c9d8a882eb..54f05894d4dd8c4a 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<minarcs to check warning is printed
+tst-mcount-overflow-ENV := GMON_OUT_PREFIX=$(objpfx)tst-mcount-overflow.data \
+                           GLIBC_TUNABLES=glibc.gmon.minarcs=51:glibc.gmon.maxarcs=50
+# Send stderr into output file because we make sure expected messages are printed
+tst-mcount-overflow-ARGS := 2>&1 1>/dev/null | cat
+ifeq ($(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)gcrt1.o
 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 bf76358d5b1aa2da..689bf80141e559ca 100644
--- a/gmon/gmon.c
+++ b/gmon/gmon.c
@@ -46,6 +46,11 @@
 #include <libc-internal.h>
 #include <not-cancel.h>
 
+#if HAVE_TUNABLES
+# define TUNABLE_NAMESPACE gmon
+# include <elf/dl-tunables.h>
+#endif
+
 #ifdef PIC
 # include <link.h>
 
@@ -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 9d4a1a50fa6ab21a..f7180fdb83399a14 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 <atomic.h>
 
+#include <not-cancel.h>
+#include <unistd.h>
+#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 b4cc3b043a2aec77..af0582a3717085b5 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 0000000000000000..27eb5538fd573a6e
--- /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
+# <https://www.gnu.org/licenses/>.
+
+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 0000000000000000..06cc93ef872eb7c1
--- /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
+   <https://www.gnu.org/licenses/>.  */
+
+/* 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 7b70e80391ee87f7..00eafcf44b562b9e 100644
--- a/manual/tunables.texi
+++ b/manual/tunables.texi
@@ -73,6 +73,9 @@ glibc.malloc.check: 0 (min: 0, max: 3)
 * Elision Tunables::  Tunables in elision subsystem
 * Hardware Capability Tunables::  Tunables that modify the hardware
 				  capabilities seen by @theglibc{}
+* gmon Tunables::  Tunables that control the gmon profiler, used in
+                   conjunction with gprof
+
 @end menu
 
 @node Tunable names
@@ -506,3 +509,59 @@ instead.
 
 This tunable is specific to i386 and x86-64.
 @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
diff --git a/sysdeps/unix/sysv/linux/aarch64/dl-tunables.list b/sysdeps/unix/sysv/linux/aarch64/dl-tunables.list
index 5c3c5292025607a1..265f82ef2be42fd0 100644
--- a/sysdeps/unix/sysv/linux/aarch64/dl-tunables.list
+++ b/sysdeps/unix/sysv/linux/aarch64/dl-tunables.list
@@ -24,3 +24,7 @@
 
 # Tunables added in RHEL 8.8.0
 @order glibc.rtld.dynamic_sort
+
+# Tunables added in RHEL 8.9.0
+@order glibc.gmon.minarcs
+@order glibc.gmon.maxarcs
diff --git a/sysdeps/unix/sysv/linux/i386/dl-tunables.list b/sysdeps/unix/sysv/linux/i386/dl-tunables.list
index b9cad4af62d9f2e5..9c1ccb86501c61e7 100644
--- a/sysdeps/unix/sysv/linux/i386/dl-tunables.list
+++ b/sysdeps/unix/sysv/linux/i386/dl-tunables.list
@@ -31,3 +31,7 @@
 
 # Tunables added in RHEL 8.8.0
 @order glibc.rtld.dynamic_sort
+
+# Tunables added in RHEL 8.9.0
+@order glibc.gmon.minarcs
+@order glibc.gmon.maxarcs
diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc64/le/dl-tunables.list b/sysdeps/unix/sysv/linux/powerpc/powerpc64/le/dl-tunables.list
index ee1e6fca95e1f2da..c8bb1a8ec0283ac8 100644
--- a/sysdeps/unix/sysv/linux/powerpc/powerpc64/le/dl-tunables.list
+++ b/sysdeps/unix/sysv/linux/powerpc/powerpc64/le/dl-tunables.list
@@ -24,3 +24,7 @@
 
 # Tunables added in RHEL 8.8.0
 @order glibc.rtld.dynamic_sort
+
+# Tunables added in RHEL 8.9.0
+@order glibc.gmon.minarcs
+@order glibc.gmon.maxarcs
diff --git a/sysdeps/unix/sysv/linux/s390/s390-64/dl-tunables.list b/sysdeps/unix/sysv/linux/s390/s390-64/dl-tunables.list
index 099e28d8f8e67944..85b3a014ffcadc45 100644
--- a/sysdeps/unix/sysv/linux/s390/s390-64/dl-tunables.list
+++ b/sysdeps/unix/sysv/linux/s390/s390-64/dl-tunables.list
@@ -23,3 +23,7 @@
 
 # Tunables added in RHEL 8.8.0
 @order glibc.rtld.dynamic_sort
+
+# Tunables added in RHEL 8.9.0
+@order glibc.gmon.minarcs
+@order glibc.gmon.maxarcs
diff --git a/sysdeps/unix/sysv/linux/x86_64/64/dl-tunables.list b/sysdeps/unix/sysv/linux/x86_64/64/dl-tunables.list
index b9cad4af62d9f2e5..9c1ccb86501c61e7 100644
--- a/sysdeps/unix/sysv/linux/x86_64/64/dl-tunables.list
+++ b/sysdeps/unix/sysv/linux/x86_64/64/dl-tunables.list
@@ -31,3 +31,7 @@
 
 # Tunables added in RHEL 8.8.0
 @order glibc.rtld.dynamic_sort
+
+# Tunables added in RHEL 8.9.0
+@order glibc.gmon.minarcs
+@order glibc.gmon.maxarcs
