Re: [PATCH dwarves 3/3] dwarf_loader: Check DW_OP_[GNU_]entry_value for possible parameter matching

Arnaldo Carvalho de Melo
2024-11-11T13:42:06Z

Adding Mark to the CC list.

On Mon, Nov 11, 2024 at 01:36:52PM +0100, Jiri Olsa wrote:
> On Mon, Nov 11, 2024 at 12:01:13AM -0800, Eduard Zingerman wrote:
> > On Sun, 2024-11-10 at 03:38 -0800, Eduard Zingerman wrote:
> >
> > [...]
> >
> > > Also, it appears there is some bug either in pahole or in libdw's
> > > implementation of dwarf_getlocation(). When I try both your patch-set
> > > and my variant there is a segfault once in a while:
> > >
> > > $ for i in $(seq 1 100); \
> > > do echo "---> $i"; \
> > > pahole -j --skip_encoding_btf_inconsistent_proto -J --btf_encode_detached=/dev/null vmlinux ; \
> > > done
> > > ---> 1
> > > ...
> > > ---> 71
> > > Segmentation fault (core dumped)
> > > ...
> > >
> > > The segfault happens only when -j (multiple threads) is passed.
> > > If pahole is built with sanitizers
> > > (passing -DCMAKE_C_FLAGS="-fsanitize=undefined,address")
> > > the stack trace looks as follows:
> >
> > Did some additional research for these SEGFAULTs.
> > Looks like all we are in trouble.
> >
> > # TLDR
> >
> > libdw is not supposed to be used in a concurrent context.
> > libdw is a part of elfutils package, the configuration flag
> > making API thread-safe is documented as experimental:
> > --enable-thread-safety enable thread safety of libraries EXPERIMENTAL
> > At-least Fedora 40 does not ship elfutils built with this flag set.
> > This colours all current parallel DWARF decoding questionable.
> >
> > # Why segfault happens
> >
> > Any references to elfutils source code are for commit [1].
> > The dwarf_getlocation() is one of a few libdw APIs that uses memory
> > allocation internally. The function dwarf_getlocation.c:__libdw_intern_expression
> > iterates over expression encodings in DWARF and allocates
> > a set of objects of type `struct loclist` and `Dwarf_Op`.
> > Pointers to allocated objects are put to a binary tree for caching,
> > see dwarf_getlocation.c:660, the call to eu_tsearch() function.
> > The eu_tsearch() is a wrapper around libc tsearch() function.
> > This wrapper provides locking for the tree,
> > but only if --enable-thread-safety was set during elfutils configuration.
> > The SEGFAULT happens inside tsearch() call because binary tree is malformed, e.g.:
> >
> > Thread 8 "pahole" received signal SIGSEGV, Segmentation fault.
> > [Switching to Thread 0x7fffd9c006c0 (LWP 2630074)]
> > 0x00007ffff7c5d200 in maybe_split_for_insert (...) at tsearch.c:228
> > 228 if (parentp != NULL && RED(DEREFNODEPTR(parentp)))
> > (gdb) bt
> > #0 0x00007ffff7c5d200 in maybe_split_for_insert (...) at tsearch.c:228
> > #1 0x00007ffff7c5d466 in __GI___tsearch (...) at tsearch.c:358
> > #2 __GI___tsearch (...) at tsearch.c:290
> > #3 0x000000000048f096 in __interceptor_tsearch ()
> > #4 0x00007ffff7f5c482 in __libdw_intern_expression (...) at dwarf_getlocation.c:660
> > #5 0x00007ffff7f5cf51 in getlocation (...) at dwarf_getlocation.c:678
> > #6 getlocation (...) at dwarf_getlocation.c:667
> > #7 dwarf_getlocation (..._ at dwarf_getlocation.c:708
> > #8 0x00000000005a2ee5 in parameter.new ()
> > #9 0x00000000005a0122 in die.process_function ()
> > #10 0x0000000000597efd in __die__process_tag ()
> > #11 0x0000000000595ad9 in die.process_unit ()
> > #12 0x0000000000595436 in die.process ()
> > #13 0x00000000005b0187 in dwarf_cus.process_cu ()
> > #14 0x00000000005afa38 in dwarf_cus.process_cu_thread ()
> > #15 0x00000000004c7b8d in asan_thread_start(void*) ()
> > #16 0x00007ffff7bda6d7 in start_thread (arg=<optimized out>) at pthread_create.c:447
> > #17 0x00007ffff7c5e60c in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:78
> > (gdb) p parentp
> > $1 = (node *) 0x50300079d2a0
> > (gdb) p *parentp
> > $2 = (node) 0x0
> >
> > glibc provides a way to validate binary tree structure.
> > For this misc/tsearch.c has to be changed to define DEBUGGING variable.
> > (I used glibc 2.39 as provided by source rpm for Fedora 40 for experiments).
> > If this is done and custom glibc is used for pahole execution,
> > the following error is reported if '-j' flag is present:
> >
> > $ pahole -j --skip_encoding_btf_inconsistent_proto -J --btf_encode_detached=/home/eddy/work/tmp/my-new.btf vmlinux
> > Fatal glibc error: tsearch.c:164 (check_tree_recurse): assertion failed: d_sofar == d_total
> > Fatal glibc error: tsearch.c:164 (check_tree_recurse): assertion failed: d_sofar == d_total
> > Aborted (core dumped)
> >
> > Executing pahole using a custom-built libdw,
> > built with --enable-thread-safety resolves the issue.
>
> could we use libdw__lock around that? but I guess we use it on other
> places as well..

Right, if you look at the cset where I intrduced libdw__lock (at the end
of this message), its a similar case, probably we should rename it to
libdw__decl_lock and introduce a libdw__getlocation_lock? This way, for
these known to be not thread safe we use libdw in serially while not
having a libdw "big lock" getting too much in the way of pahole's
parallel operation?

I'll take a closer look at Edward's analysis and the code, as well as
conversations I had with Mark Wieelard at the time.

- Arnaldo

$ git show 1caed1c443d4a0dc5
commit 1caed1c443d4a0dc5794f21954be7d5ae0396c90
Author: Arnaldo Carvalho de Melo <acme@redhat.com>
Date: Thu Jul 8 16:07:50 2021 -0300

dwarf_loader: Add a lock around dwarf_decl_file() and dwarf_decl_line() calls

As this ends up racing on a tsearch() call, probably for some libdw
cache that gets updated/lookedup in concurrent pahole threads (-j N).

This cures the following, a patch for libdw will be cooked up and sent.

(gdb) run -j -I -F dwarf vmlinux > /dev/null
Starting program: /var/home/acme/git/pahole/build/pahole -j -I -F dwarf vmlinux > /dev/null
warning: Expected absolute pathname for libpthread in the inferior, but got .gnu_debugdata for /lib64/libpthread.so.0.
warning: File "/usr/lib64/libthread_db-1.0.so" auto-loading has been declined by your `auto-load safe-path' set to "$debugdir:$datadir/auto-load".
warning: Unable to find libthread_db matching inferior's thread library, thread debugging will not be available.
[New LWP 844789]
[New LWP 844790]
[New LWP 844791]
[New LWP 844792]
[New LWP 844793]
[New LWP 844794]
[New LWP 844795]
[New LWP 844796]
[New LWP 844797]
[New LWP 844798]
[New LWP 844799]
[New LWP 844800]
[New LWP 844801]
[New LWP 844802]
[New LWP 844803]
[New LWP 844804]
[New LWP 844805]
[New LWP 844806]
[New LWP 844807]
[New LWP 844808]
[New LWP 844809]
[New LWP 844810]
[New LWP 844811]
[New LWP 844812]
[New LWP 844813]
[New LWP 844814]

Thread 2 "pahole" received signal SIGSEGV, Segmentation fault.
[Switching to LWP 844789]
0x00007ffff7dfa321 in ?? () from /lib64/libc.so.6
(gdb) bt
#0 0x00007ffff7dfa321 in ?? () from /lib64/libc.so.6
#1 0x00007ffff7dfa4bb in ?? () from /lib64/libc.so.6
#2 0x00007ffff7f5eaa6 in __libdw_getsrclines (dbg=0x4a7f90, debug_line_offset=10383710, comp_dir=0x7ffff3c29f01 "/var/home/acme/git/build/v5.13.0-rc6+", address_size=address_size@entry=8, linesp=linesp@entry=0x7fffcfe04ba0, filesp=filesp@entry=0x7fffcfe04ba8)
at dwarf_getsrclines.c:1129
#3 0x00007ffff7f5ed14 in dwarf_getsrclines (cudie=cudie@entry=0x7fffd210caf0, lines=lines@entry=0x7fffd210cac0, nlines=nlines@entry=0x7fffd210cac8) at dwarf_getsrclines.c:1213
#4 0x00007ffff7f64883 in dwarf_decl_file (die=<optimized out>) at dwarf_decl_file.c:66
#5 0x0000000000425f24 in tag__init (tag=0x7fff0421b710, cu=0x7fffcc001e40, die=0x7fffd210cd30) at /var/home/acme/git/pahole/dwarf_loader.c:476
#6 0x00000000004262ec in namespace__init (namespace=0x7fff0421b710, die=0x7fffd210cd30, cu=0x7fffcc001e40, conf=0x475600 <conf_load>) at /var/home/acme/git/pahole/dwarf_loader.c:576
#7 0x00000000004263ac in type__init (type=0x7fff0421b710, die=0x7fffd210cd30, cu=0x7fffcc001e40, conf=0x475600 <conf_load>) at /var/home/acme/git/pahole/dwarf_loader.c:595
#8 0x00000000004264d1 in type__new (die=0x7fffd210cd30, cu=0x7fffcc001e40, conf=0x475600 <conf_load>) at /var/home/acme/git/pahole/dwarf_loader.c:614
#9 0x0000000000427ba6 in die__create_new_typedef (die=0x7fffd210cd30, cu=0x7fffcc001e40, conf=0x475600 <conf_load>) at /var/home/acme/git/pahole/dwarf_loader.c:1212
#10 0x0000000000428df5 in __die__process_tag (die=0x7fffd210cd30, cu=0x7fffcc001e40, top_level=1, fn=0x45cee0 <__FUNCTION__.10> "die__process_unit", conf=0x475600 <conf_load>) at /var/home/acme/git/pahole/dwarf_loader.c:1823
#11 0x0000000000428ea1 in die__process_unit (die=0x7fffd210cd30, cu=0x7fffcc001e40, conf=0x475600 <conf_load>) at /var/home/acme/git/pahole/dwarf_loader.c:1848
#12 0x0000000000429e45 in die__process (die=0x7fffd210ce20, cu=0x7fffcc001e40, conf=0x475600 <conf_load>) at /var/home/acme/git/pahole/dwarf_loader.c:2311
#13 0x0000000000429ecb in die__process_and_recode (die=0x7fffd210ce20, cu=0x7fffcc001e40, conf=0x475600 <conf_load>) at /var/home/acme/git/pahole/dwarf_loader.c:2326
#14 0x000000000042a9d6 in dwarf_cus__create_and_process_cu (dcus=0x7fffffffddc0, cu_die=0x7fffd210ce20, pointer_size=8 '\b') at /var/home/acme/git/pahole/dwarf_loader.c:2644
#15 0x000000000042ab28 in dwarf_cus__process_cu_thread (arg=0x7fffffffddc0) at /var/home/acme/git/pahole/dwarf_loader.c:2687
#16 0x00007ffff7ed6299 in start_thread () from /lib64/libpthread.so.0
#17 0x00007ffff7dfe353 in ?? () from /lib64/libc.so.6
(gdb)
(gdb) fr 2
1085
(gdb) list files_lines_compare
1086 static int
1087 files_lines_compare (const void *p1, const void *p2)
1088 {
1089 const struct files_lines_s *t1 = p1;
1090 const struct files_lines_s *t2 = p2;
1091
1092 if (t1->debug_line_offset < t2->debug_line_offset)
(gdb)
1093 return -1;
1094 if (t1->debug_line_offset > t2->debug_line_offset)
1095 return 1;
1096
1097 return 0;
1098 }
1099
1100 int
1101 internal_function
1102 __libdw_getsrclines (Dwarf *dbg, Dwarf_Off debug_line_offset,
(gdb) list __libdw_getsrclines
1100 int
1101 internal_function
1102 __libdw_getsrclines (Dwarf *dbg, Dwarf_Off debug_line_offset,
1103 const char *comp_dir, unsigned address_size,
1104 Dwarf_Lines **linesp, Dwarf_Files **filesp)
1105 {
1106 struct files_lines_s fake = { .debug_line_offset = debug_line_offset };
1107 struct files_lines_s **found = tfind (&fake, &dbg->files_lines,
1108 files_lines_compare);
1109 if (found == NULL)
(gdb)
1110 {
1111 Elf_Data *data = __libdw_checked_get_data (dbg, IDX_debug_line);
1112 if (data == NULL
1113 || __libdw_offset_in_section (dbg, IDX_debug_line,
1114 debug_line_offset, 1) != 0)
1115 return -1;
1116
1117 const unsigned char *linep = data->d_buf + debug_line_offset;
1118 const unsigned char *lineendp = data->d_buf + data->d_size;
1119
(gdb)
1120 struct files_lines_s *node = libdw_alloc (dbg, struct files_lines_s,
1121 sizeof *node, 1);
1122
1123 if (read_srclines (dbg, linep, lineendp, comp_dir, address_size,
1124 &node->lines, &node->files) != 0)
1125 return -1;
1126
1127 node->debug_line_offset = debug_line_offset;
1128
1129 found = tsearch (node, &dbg->files_lines, files_lines_compare);
(gdb)

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>