329008fdf1
Summary: Lldb support base address selection entries in location lists was broken for a long time. This wasn't noticed until llvm started producing these kinds of entries more frequently with r374600. In r374769, I made a quick patch which added sufficient support for them to get the test suite to pass. However, I did not fully understand how this code operates, and so the fix was not complete. Specifically, what was lacking was the ability to handle modules which were not loaded at their preferred load address (for instance, due to ASLR). Now that I better understand how this code works, I've come to the conclusion that the current setup does not provide enough information to correctly process these entries. In the current setup the location lists were parameterized by two addresses: - the distance of the function start from the start of the compile unit. The purpose of this was to make the location ranges relative to the start of the function. - the actual address where the function was loaded at. With this the function-start-relative ranges can be translated to actual memory locations. The reason for the two values, instead of just one (the load bias) is (I think) MachO, where the debug info in the object files will appear to be relative to the address zero, but the actual code it refers to can be moved and reordered by the linker. This means that the location lists need to be "linked" to reflect the locations in the actual linked file. These two bits of information were enough to correctly process location lists which do not contain base address selection entries (and so all entries are relative to the CU base). However, they don't work with them because, in theory two base address can be completely unrelated (as can happen for instace with hot/cold function splitting, where the linker can reorder the two pars arbitrarily). To fix that, I split the first parameter into two: - the compile unit base address - the function start address, as is known in the object file The new algorithm becomes: - the location lists are processed as they were meant to be processed. The CU base address is used as the initial base address value. Base address selection entries can set a new base. - the difference between the "file" and "load" function start addresses is used to compute the load bias. This value is added to the final ranges to get the actual memory location. This algorithm is correct for non-MachO debug info, as there the location lists correctly describe the code in the final executable, and the dynamic linker can just move the entire module, not pieces of it. It will also be correct for MachO if the static linker preserves relative positions of the various parts of the location lists -- I don't know whether it actually does that, but judging by the lack of base address selection support in dsymutil and lldb, this isn't something that has come up in the past. I add a test case which simulates the ASLR scenario and demonstrates that base address selection entries now work correctly here. Reviewers: JDevlieghere, aprantl, clayborg Subscribers: dblaikie, lldb-commits Tags: #lldb Differential Revision: https://reviews.llvm.org/D70532
130 lines
5.9 KiB
ArmAsm
130 lines
5.9 KiB
ArmAsm
# This test checks the handling of location lists in the case when the module is
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# not loaded at the address at which it was linked (as happens with ASLR for
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# instance).
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# REQUIRES: x86
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# RUN: yaml2obj %S/Inputs/debug_loc-aslr.yaml >%t.dmp
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# RUN: llvm-mc --triple=x86_64-pc-linux --filetype=obj %s >%t.o
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# RUN: %lldb -c %t.dmp -o "image add %t.o" \
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# RUN: -o "image load --file %t.o --slide 0x470000" \
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# RUN: -o "thread info" -o "frame variable" -o exit | FileCheck %s
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# CHECK: thread #1: tid = 16001, 0x0000000000470001 {{.*}}`_start
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# CHECK: (int) x = 47
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# CHECK: (int) y = 74
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.text
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.globl _start
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_start:
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nop
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retq
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.Lstart_end:
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.section .debug_loc,"",@progbits
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# This location list implicitly uses the base address of the compile unit.
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.Ldebug_loc0:
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.quad _start-_start
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.quad .Lstart_end-_start
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.short 3 # Loc expr size
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.byte 8 # DW_OP_const1u
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.byte 47
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.byte 159 # DW_OP_stack_value
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.quad 0
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.quad 0
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# This is an equivalent location list to the first one, but here the base
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# address is set explicitly.
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.Ldebug_loc1:
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.quad -1
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.quad _start
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.quad _start-_start
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.quad .Lstart_end-_start
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.short 3 # Loc expr size
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.byte 8 # DW_OP_const1u
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.byte 74
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.byte 159 # DW_OP_stack_value
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.quad 0
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.quad 0
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.section .debug_abbrev,"",@progbits
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.byte 1 # Abbreviation Code
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.byte 17 # DW_TAG_compile_unit
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.byte 1 # DW_CHILDREN_yes
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.byte 37 # DW_AT_producer
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.byte 8 # DW_FORM_string
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.byte 19 # DW_AT_language
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.byte 5 # DW_FORM_data2
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.byte 17 # DW_AT_low_pc
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.byte 1 # DW_FORM_addr
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.byte 18 # DW_AT_high_pc
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.byte 6 # DW_FORM_data4
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.byte 0 # EOM(1)
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.byte 0 # EOM(2)
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.byte 2 # Abbreviation Code
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.byte 46 # DW_TAG_subprogram
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.byte 1 # DW_CHILDREN_yes
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.byte 17 # DW_AT_low_pc
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.byte 1 # DW_FORM_addr
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.byte 18 # DW_AT_high_pc
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.byte 6 # DW_FORM_data4
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.byte 3 # DW_AT_name
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.byte 8 # DW_FORM_string
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.byte 0 # EOM(1)
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.byte 0 # EOM(2)
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.byte 4 # Abbreviation Code
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.byte 52 # DW_TAG_variable
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.byte 0 # DW_CHILDREN_no
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.byte 2 # DW_AT_location
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.byte 23 # DW_FORM_sec_offset
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.byte 3 # DW_AT_name
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.byte 8 # DW_FORM_string
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.byte 73 # DW_AT_type
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.byte 19 # DW_FORM_ref4
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.byte 0 # EOM(1)
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.byte 0 # EOM(2)
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.byte 6 # Abbreviation Code
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.byte 36 # DW_TAG_base_type
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.byte 0 # DW_CHILDREN_no
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.byte 3 # DW_AT_name
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.byte 8 # DW_FORM_string
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.byte 62 # DW_AT_encoding
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.byte 11 # DW_FORM_data1
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.byte 11 # DW_AT_byte_size
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.byte 11 # DW_FORM_data1
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.byte 0 # EOM(1)
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.byte 0 # EOM(2)
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.byte 0 # EOM(3)
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.section .debug_info,"",@progbits
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.Lcu_begin0:
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.long .Ldebug_info_end0-.Ldebug_info_start0 # Length of Unit
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.Ldebug_info_start0:
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.short 4 # DWARF version number
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.long .debug_abbrev # Offset Into Abbrev. Section
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.byte 8 # Address Size (in bytes)
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.byte 1 # Abbrev [1] 0xb:0x6a DW_TAG_compile_unit
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.asciz "Hand-written DWARF" # DW_AT_producer
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.short 12 # DW_AT_language
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.quad _start # DW_AT_low_pc
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.long .Lstart_end-_start # DW_AT_high_pc
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.byte 2 # Abbrev [2] 0x2a:0x43 DW_TAG_subprogram
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.quad _start # DW_AT_low_pc
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.long .Lstart_end-_start # DW_AT_high_pc
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.asciz "_start" # DW_AT_name
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.byte 4 # Abbrev [4] 0x52:0xf DW_TAG_variable
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.long .Ldebug_loc0 # DW_AT_location
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.asciz "x" # DW_AT_name
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.long .Lint # DW_AT_type
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.byte 4 # Abbrev [4] 0x52:0xf DW_TAG_variable
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.long .Ldebug_loc1 # DW_AT_location
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.asciz "y" # DW_AT_name
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.long .Lint # DW_AT_type
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.byte 0 # End Of Children Mark
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.Lint:
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.byte 6 # Abbrev [6] 0x6d:0x7 DW_TAG_base_type
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.asciz "int" # DW_AT_name
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.byte 5 # DW_AT_encoding
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.byte 4 # DW_AT_byte_size
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.byte 0 # End Of Children Mark
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.Ldebug_info_end0:
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