[Crash-utility] Re: [PATCH] gdb: fix the "p" command incorrectly print the value of a global variable
Lianbo,
Thank you for your work.
Some objects format may potentially support copy relocations, but
currently the maybe_copied is always initialized to 0 in the symbol().
And the type is 'mst_file_bss', not always the 'mst_bss' or
'mst_data'
in the lookup_minimal_symbol_linkage(). For example:
(gdb) p *msymbol
$42 = {<general_symbol_info> = {m_name = 0x349812f "test_no_static",
value = {ivalue = 8, block = 0x8,
bytes = 0x8 <error: Cannot access memory at address 0x8>, address = 8,
common_block = 0x8, chain = 0x8}, language_specific = {
obstack = 0x0, demangled_name = 0x0}, m_language = language_auto, ada_mangled = 0,
section = 20}, size = 4,
filename = 0x6db3440 "test_sanity.c", type = mst_file_bss, created_by_gdb =
0, target_flag_1 = 0, target_flag_2 = 0, has_size = 1,
maybe_copied = 0, name_set = 1, hash_next = 0x0, demangled_hash_next = 0x0}
The current description lacks explanation of when this issue
occurs. Please write that the issue occurs when the corresponding
kernel is built with CONFIG_CALL_DEPTH_TRACKING=y.
It would also be good to describe the fact that the issue occurs at
least on RHEL9 kernel.
This causes a problem that the 'p' command can not work well
as
expected, and always gets an error:
crash> mod -s test_sanity /home/test_sanity.ko
MODULE NAME BASE SIZE OBJECT FILE
ffffffffc1084040 test_sanity ffffffffc1082000 16384
/home/test_sanity.ko
crash> p test_no_static
p: gdb request failed: p test_no_static
crash>
With the patch:
crash> mod -s test_sanity /home/test_sanity.ko
MODULE NAME BASE SIZE OBJECT FILE
ffffffffc1084040 test_sanity ffffffffc1082000 16384
/home/test_sanity.ko
crash> p test_no_static
test_no_static = $1 = 5
crash>
It's correct that p command doesn't work as expected, but it doesn't
always result in some error. This issue is failure of calculating
relocated address of static symbols. If the calculated address happens
to be the address where read can be successfull, it doesn't result in
read error but outputs some bogus value.
To make this clear, I think it's better to set debug level 4 and to
have p command output calculated virtual address as debug messages.
For example:
crash> sym -M | grep -E " test_no"
ffffffffc0da7580 (B) test_no
ffffffffc0da7584 (b) test_no_static
crash> set debug 4
debug: 4
crash> p test_no
p: per_cpu_symbol_search(test_no): NULL
test_no = <readmem: ffffffffc0da7580, KVADDR, "gdb_readmem callback", 4,
(ROE), 560d2d483400>
<read_diskdump: addr: ffffffffc0da7580 paddr: 10b263580 cnt: 4>
$3 = 5
crash> p test_no_static
p: per_cpu_symbol_search(test_no_static): NULL
test_no_static = <readmem: ffffffffc0d9f004, KVADDR, "gdb_readmem
callback", 4, (ROE), 560d2dc9b100>
<read_diskdump: addr: ffffffffc0d9f004 paddr: 108bfc004 cnt: 4>
$4 = -1869574000