[Bug backtrace/27987] aarch64 prologue unwinder does not correctly handle -fpatchable-function-entry=2 flag

["luis.machado at linaro dot org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27987

Luis Machado <luis.machado at linaro dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2021-06-29

--- Comment #4 from Luis Machado <luis.machado at linaro dot org> ---
On a brief look, it may not be the prologue analyzer's fault. When using
"-fpatchable-function-entry=N,M", the entry point of the function is populated
with N nops, and the entry point will point at the (M - 1)-th nop. The function
symbol's address is adjusted to reflect this, but the DWARF information for the
function symbol is not.

objdump output:

00000000000010ec <t_small_values>:
    10ec:       d503201f        nop
    10f0:       d503201f        nop
    10f4:       d503201f        nop
    10f8:       d503201f        nop
    10fc:       d100c3ff        sub     sp, sp, #0x30


DWARF:

 <1><c63>: Abbrev Number: 26 (DW_TAG_subprogram)
    <c64>   DW_AT_external    : 1
    <c64>   DW_AT_name        : (indirect string, offset: 0x1e77):
t_small_values
    <c68>   DW_AT_decl_file   : 1
    <c69>   DW_AT_decl_line   : 288
    <c6b>   DW_AT_decl_column : 1
    <c6c>   DW_AT_type        : <0x3f>
    <c70>   DW_AT_low_pc      : 0x10fc
    <c78>   DW_AT_high_pc     : 0xb8
    <c80>   DW_AT_frame_base  : 1 byte block: 9c        (DW_OP_call_frame_cfa)
    <c82>   DW_AT_GNU_all_call_sites: 1
    <c82>   DW_AT_sibling     : <0xd27>


Notice how DW_AT_low_pc points at 0x10fc, the first real instruction of the
function.

I'm not sure if this is by design, but anything coming before the real entry
point to the function will be ignored by GDB's prologue analyzer.

GDB may need to handle patchable functions in a better way, but this may hint
at a problem in GCC's DWARF generation. If GCC didn't choose to not emit the
adjusted DT_AT_low_pc by design, that is.

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