On Fri, Jul 15, 2016 at 3:59 AM Alexei Starovoitov < alexei.starovoi...@gmail.com> wrote:
> On Wed, Jul 13, 2016 at 4:17 PM, Kyle Laracey via iovisor-dev > <iovisor-dev@lists.iovisor.org> wrote: > > > > An additional issue brought up on the call (I apologize for forgetting by > > whom) was that any comparisons on long strings would bloat the program, > > perhaps beyond the maximum allowed program size. Would the solution to > this > > be to have something like another BPF helper function in the kernel? > > the idea we discussed is to introduce string map similar to stackmap > where different strings are referenced by id. The helper would be able > to push user or kernel strings into this map. > That should solve https://github.com/iovisor/bcc/issues/607 > What is your use case that needs strings? > To come up with good kernel design we need to > categorize all use cases that need strings. > Both the trace and argdist tools collect strings from tracepoints, uprobes/kprobes, and USDT tracepoints. These strings are displayed to the user (trace) [1] or used as keys for histograms and event frequency counters (argdist) [2]. The length of the string is usually not known in advance, so something like strcpy would be very useful. The proposed string map solution would work as long as the kernel helper would be able to support two modes of operation: read string until null terminator, and read N characters. We will also need a helper that can actually retrieve the string based on its id. [1] Examples of tracing string values: file names being opened, JVM thread names starting/finishing. [2] Examples of frequency counting string values: web server URLs being hit (e.g. in Node.js USDT probes), exec() command lines, JVM method names being executed.
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