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fix CO-RE read for cast structs

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varun-r-mallya
2025-11-22 01:47:25 +05:30
parent 84507b8b98
commit 6f25c554a9
4 changed files with 50 additions and 16 deletions
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36
pythonbpf/vmlinux_parser/vmlinux_exports_handler.py
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@ -115,11 +115,11 @@ class VmlinuxHandler:
struct_name = python_type.__name__
globvar_ir, field_data = self.get_field_type(struct_name, field_name)
# Handle cast struct field access (use standard GEP)
# Handle cast struct field access (use bpf_probe_read_kernel)
# Load the struct pointer from the local variable
struct_ptr = builder.load(var_info.var)
# Use standard GEP for non-context struct field access
# Use bpf_probe_read_kernel for non-context struct field access
field_value = self.load_struct_field(
builder, struct_ptr, globvar_ir, field_data, struct_name
)
@ -133,7 +133,7 @@ class VmlinuxHandler:
builder, struct_ptr_int, offset_global, field_data, struct_name=None
):
"""
Generate LLVM IR to load a field from a regular (non-context) struct using standard GEP.
Generate LLVM IR to load a field from a regular (non-context) struct using bpf_probe_read_kernel.
Args:
builder: llvmlite IRBuilder instance
@ -159,7 +159,8 @@ class VmlinuxHandler:
inbounds=False,
)
# Determine the appropriate IR type based on field information
# Determine the appropriate field size based on field information
field_size_bytes = 8 # Default to 8 bytes (64-bit)
int_width = 64 # Default to 64-bit
needs_zext = False
@ -172,7 +173,7 @@ class VmlinuxHandler:
if field_size_bits in [8, 16, 32, 64]:
int_width = field_size_bits
logger.info(f"Determined field size: {int_width} bits")
logger.info(f"Determined field size: {int_width} bits ({field_size_bytes} bytes)")
# Special handling for struct_xdp_md i32 fields
if struct_name == "struct_xdp_md" and int_width == 32:
@ -195,16 +196,31 @@ class VmlinuxHandler:
field_data.ctype_complex_type, ctypes._Pointer
):
int_width = 64 # Pointers are always 64-bit
field_size_bytes = 8
logger.info("Field is a pointer type, using 64 bits")
else:
logger.warning("Complex vmlinux field type, using default 64 bits")
# Bitcast to appropriate pointer type based on determined width
ptr_type = ir.PointerType(ir.IntType(int_width))
typed_ptr = builder.bitcast(field_ptr, ptr_type)
# Allocate local storage for the field value
local_storage = builder.alloca(ir.IntType(int_width))
local_storage_i8_ptr = builder.bitcast(local_storage, i8_ptr_type)
# Load the value
value = builder.load(typed_ptr)
# Use bpf_probe_read_kernel to safely read the field
# This generates:
# %gep = getelementptr i8, ptr %struct_ptr, i64 %offset (already done above as field_ptr)
# %passed = tail call ptr @llvm.bpf.passthrough.p0.p0(i32 2, ptr %gep)
# %result = call i64 inttoptr (i64 113 to ptr)(ptr %local_storage, i32 %size, ptr %passed)
from pythonbpf.helper import emit_probe_read_kernel_call
result = emit_probe_read_kernel_call(
builder,
local_storage_i8_ptr,
field_size_bytes,
field_ptr
)
# Load the value from local storage
value = builder.load(local_storage)
# Zero-extend i32 to i64 if needed
if needs_zext: