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When a processor faults/is interrupted/gets an exception, it will stop
running code and jump to an exception catcher routine. Most processors
will store the pc that was executing in a system register, and the
catcher functions have special instructions to retrieve that & possibly
other registers. It may then save those values to stack, and the author
can add .cfi directives to tell lldb's unwinder where to find those
saved values.
ARM Cortex-M (microcontroller) processors have a simpler mechanism where
a fixed set of registers are saved to the stack on an exception, and a
unique value is put in the link register to indicate to the caller that
this has taken place. No special handling needs to be written into the
exception catcher, unless it wants to inspect these preserved values.
And it is possible for a general stack walker to walk the stack with no
special knowledge about what the catch function does.
This patch adds an Architecture plugin method to allow an Architecture
to override/augment the UnwindPlan that lldb would use for a stack
frame, given the contents of the return address register. It resembles a
feature where the LanguageRuntime can replace/augment the unwind plan
for a function, but it is doing it at offset by one level. The
LanguageRuntime is looking at the local register context and/or symbol
name to decide if it will override the unwind rules. For the Cortex-M
exception unwinds, we need to modify THIS frame's unwind plan if the
CALLER's LR had a specific value. RegisterContextUnwind has to retrieve
the caller's LR value before it has completely decided on the UnwindPlan
it will use for THIS stack frame.
This does mean that we will need one additional read of stack memory
than we currently do when unwinding, on Armv7 Cortex-M targets. The
unwinder walks the stack lazily, as stack frames are requested, and so
now if you ask for 2 stack frames, we will read enough stack to walk 2
frames, plus we will read one extra word of memory, the spilled LR value
from the stack. In practice, with 512-byte memory cache reads, this is
unlikely to be a real performance hit.
This PR includes a test with a yaml corefile description and a JSON
ObjectFile, incorporating all of the necessary stack memory and symbol
names from a real debug session I worked on. The architectural default
unwind plans are used for all stack frames except the 0th because
there's no instructions for the functions, and no unwind info. I may
need to add an encoding of unwind fules to ObjectFileJSON in the future
as we create more test cases like this.
This PR depends on the yaml2macho-core utility from
https://github.com/llvm/llvm-project/pull/153911 to run its API test.
rdar://110663219
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Sections now support specifying:
- user IDs
- file offset/size
- alignment
- flags
- bool values for fake, encrypted and thread specific sections
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This patch adds the ability to create subsections in a section and
allows permissions to be specified.
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ObjectFileJSON sections didn't work, they were set to zero all of the
time. Fixed the bug and fixed the test to ensure it was testing real
values.
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This patch adds support for creating modules from JSON object files.
This is necessary for the crashlog use case where we don't have either a
module or a symbol file. In that case the ObjectFileJSON serves as both.
The patch adds support for an object file type (i.e. executable, shared
library, etc). It also adds the ability to specify sections, which is
necessary in order specify symbols by address. Finally, this patch
improves error handling and fixes a bug where we wouldn't read more than
the initial 512 bytes in GetModuleSpecifications.
Differential revision: https://reviews.llvm.org/D148062
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Introduce a new object and symbol file format with the goal of mapping
addresses to symbol names. I'd like to think of is as an extremely
simple textual symtab. The file format consists of a triple, a UUID and
a list of symbols. JSON is used for the encoding, but that's mostly an
implementation detail. The goal of the format was to be simple and human
readable.
The new file format is motivated by two use cases:
- Stripped binaries: when a binary is stripped, you lose the ability to
do thing like setting symbolic breakpoints. You can keep the
unstripped binary around, but if all you need is the stripped
symbols then that's a lot of overhead. Instead, we could save the
stripped symbols to a file and load them in the debugger when
needed. I want to extend llvm-strip to have a mode where it emits
this new file format.
- Interactive crashlogs: with interactive crashlogs, if we don't have
the binary or the dSYM for a particular module, we currently show an
unnamed symbol for those frames. This is a regression compared to the
textual format, that has these frames pre-symbolicated. Given that
this information is available in the JSON crashlog, we need a way to
tell LLDB about it. With the new symbol file format, we can easily
synthesize a symbol file for each of those modules and load them to
symbolicate those frames.
Here's an example of the file format:
{
"triple": "arm64-apple-macosx13.0.0",
"uuid": "36D0CCE7-8ED2-3CA3-96B0-48C1764DA908",
"symbols": [
{
"name": "main",
"type": "code",
"size": 32,
"address": 4294983568
},
{
"name": "foo",
"type": "code",
"size": 8,
"address": 4294983560
}
]
}
Differential revision: https://reviews.llvm.org/D145180
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