trace, argdist: STRCMP helper function #772
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`trace` filters and print expressions can now use the
magic STRCMP helper function to compare strings. The first
string must be a compile-time constant literal string,
such as "test", and the second string can be determined at
runtime (e.g., from a function argument). The codegen for
STRCMP is on a case-by-case basis for each literal string,
and it generates an inline function with a constant-length
loop that compares the string's characters. This is a
decent workaround until we get something more reasonable
from the kernel side, such as a `bpf_strcmp` helper.
Usage example:
```
trace 'p:c:open (STRCMP("test.txt", arg1)) "%s", arg1'
``
argdist filter expressions can now use the STRCMP helper
function to compare strings. The first string must be a
compile-time constant literal string, and the second string
can be determined at runtime. This is a workaround until
BPF introduces a kernel builtin for strcmp.
Example:
```
argdist -H 'r:c:open(char *file):u64:$latency:STRCMP("test.txt",file)'
```
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Neat workaround. I'd be inclined to put a comment above it to say "# this is a workaround implementation of strcmp(). In the future this might get replaced. See #691." We can see how painful there workarounds are in practice. They make be good enough... |
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I'm happy to merge; I don't know if @4ast / @drzaeus77 wanted to comment, or at least know we're trying out this workaround. :) |
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lgtm |
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Hacky solution, but works: STRCMP is replaced by a static inline function call that compares a runtime string to a compile-time literal constant. This is good enough to unroll so we don't need a backward jump, which is forbidden by BPF. Quite useful examples can arise, such as tracing opens to a specific file, aggregating operations on a specific device, and so on.
/cc @brendangregg