On s390x-linux, I ran into: ... (gdb) PASS: gdb.base/readnever.exp: continue backtrace^M #0 0x000000000100061a in fun_three ()^M #1 0x000000000100067a in fun_two ()^M #2 0x000003fffdfa9470 in ?? ()^M Backtrace stopped: frame did not save the PC^M (gdb) FAIL: gdb.base/readnever.exp: backtrace ... This seems to be related to the way this function handles saving fp (r11) and sp (r15): ... 0000000001000608 <fun_three>: 1000608: b3 c1 00 2b ldgr %f2,%r11 100060c: b3 c1 00 0f ldgr %f0,%r15 1000610: e3 f0 ff 50 ff 71 lay %r15,-176(%r15) 1000616: b9 04 00 bf lgr %r11,%r15 100061a: b9 04 00 12 lgr %r1,%r2 100061e: b9 04 00 23 lgr %r2,%r3 1000622: e3 40 b0 a0 00 24 stg %r4,160(%r11) 1000628: 50 10 b0 ac st %r1,172(%r11) 100062c: 18 12 lr %r1,%r2 100062e: 42 10 b0 ab stc %r1,171(%r11) 1000632: 07 00 nopr 1000634: b3 cd 00 b2 lgdr %r11,%f2 1000638: b3 cd 00 f0 lgdr %r15,%f0 100063c: 07 fe br %r14 100063e: 07 07 nopr %r7 ...
Created attachment 15821 [details] tentative patch
https://sourceware.org/pipermail/gdb-patches/2024-December/213821.html
The master branch has been updated by Tom de Vries <vries@sourceware.org>: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=a64f365e6406bfe42feef3e64d88dfff11dddd5a commit a64f365e6406bfe42feef3e64d88dfff11dddd5a Author: Tom de Vries <tdevries@suse.de> Date: Thu Jan 9 14:32:19 2025 +0100 [gdb/tdep] Fix gdb.base/readnever.exp on s390x On s390x-linux, I run into: ... (gdb) backtrace #0 0x000000000100061a in fun_three () #1 0x000000000100067a in fun_two () #2 0x000003fffdfa9470 in ?? () Backtrace stopped: frame did not save the PC (gdb) FAIL: gdb.base/readnever.exp: backtrace ... This is really due to a problem handling the fun_three frame. When generating a backtrace from fun_two, everying looks ok: ... $ gdb -readnever -q -batch outputs/gdb.base/readnever/readnever \ -ex "b fun_two" \ -ex run \ -ex bt ... #0 0x0000000001000650 in fun_two () #1 0x00000000010006b6 in fun_one () #2 0x00000000010006ee in main () ... For reference the frame info with debug info (without -readnever) looks like this: ... $ gdb -q -batch outputs/gdb.base/readnever/readnever \ -ex "b fun_three" \ -ex run \ -ex "info frame" ... Stack level 0, frame at 0x3fffffff140: pc = 0x1000632 in fun_three (readnever.c:20); saved pc = 0x100067a called by frame at 0x3fffffff1f0 source language c. Arglist at 0x3fffffff140, args: a=10, b=49 '1', c=0x3fffffff29c Locals at 0x3fffffff140, Previous frame's sp in v0 ... But with -readnever, like this instead: ... Stack level 0, frame at 0x0: pc = 0x100061a in fun_three; saved pc = 0x100067a called by frame at 0x3fffffff140 Arglist at 0xffffffffffffffff, args: Locals at 0xffffffffffffffff, Previous frame's sp in r15 ... An obvious difference is the "Previous frame's sp in" v0 vs. r15. Looking at the code: ... 0000000001000608 <fun_three>: 1000608: b3 c1 00 2b ldgr %f2,%r11 100060c: b3 c1 00 0f ldgr %f0,%r15 1000610: e3 f0 ff 50 ff 71 lay %r15,-176(%r15) 1000616: b9 04 00 bf lgr %r11,%r15 ... it becomes clear what is going on. This is an unusual prologue. Rather than saving r11 (frame pointer) and r15 (stack pointer) to stack, instead they're saved into call-clobbered floating point registers. [ For reference, this is the prologue of fun_two: ... 0000000001000640 <fun_two>: 1000640: eb bf f0 58 00 24 stmg %r11,%r15,88(%r15) 1000646: e3 f0 ff 50 ff 71 lay %r15,-176(%r15) 100064c: b9 04 00 bf lgr %r11,%r15 ... where the first instruction stores registers r11 to r15 to stack. ] Gdb fails to properly analyze the prologue, which causes the problems getting the frame info. Fix this by: - adding handling of the ldgr insn [1] in s390_analyze_prologue, and - recognizing the insn as saving a register in s390_prologue_frame_unwind_cache. This gets us instead: ... Stack level 0, frame at 0x0: pc = 0x100061a in fun_three; saved pc = 0x100067a called by frame at 0x3fffffff1f0 Arglist at 0xffffffffffffffff, args: Locals at 0xffffffffffffffff, Previous frame's sp in f0 ... and: ... (gdb) backtrace^M #0 0x000000000100061a in fun_three ()^M #1 0x000000000100067a in fun_two ()^M #2 0x00000000010006b6 in fun_one ()^M #3 0x00000000010006ee in main ()^M (gdb) PASS: gdb.base/readnever.exp: backtrace ... Tested on s390x-linux. PR tdep/32417 Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32417 Approved-By: Andreas Arnez <arnez@linux.ibm.com> [1] https://www.ibm.com/support/pages/sites/default/files/2021-05/SA22-7871-10.pdf
Fixed.