Locating a known source function in a stripped binary is a prerequisite for many security and software engineering tasks, including Software Composition Analysis (SCA) false-positive elimination, patch presence verification, malware analysis, code plagiarism detection, and license compliance auditing. We formalize this need as \emph{source-to-binary function localization}: given the source code of a target function and its encompassing source package, determine whether the function is present in a stripped binary and, if so, report its address. Two fundamental challenges arise: \emph{cross-modal alignment}, as source code and stripped binary reside in vastly different representation spaces; and \emph{similar function disambiguation}, as compilation erases the symbolic features that distinguish functionally similar functions.

We present XLoc, a recall-then-verify framework built on two insights. First, cross-modal alignment does not require costly and error-prone compilation; it only demands token-level alignment, a process that can be reliably approximated. Second, the information needed to disambiguate similar functions is already available on the source side and can be extracted ahead of time to guide verification. Building on these insights, XLoc implements a multi-stage recall module in which an LLM transforms source code into pseudo-decompiled representations aligned with binary decompilation output, bridging the cross-modal gap. For verification, XLoc identifies potentially confusing similar functions, extracts differential summaries, and uses them to guide the verification process toward the specific distinguishing evidence for each candidate, producing definitive accept/reject verdicts rather than similarity rankings.

We evaluate XLoc on two complementary datasets spanning 196 CVEs, 480 vulnerable functions, and 756 binaries. XLoc achieves up to $84.4%$ localization accuracy ($4.2\times$ over the best baseline) and $\text{HM}=87.1%$ for positive/negative discrimination (vs. $35.1%$ for the best baseline). These results demonstrate that XLoc can locate target functions with high accuracy, reliably discriminate between positive and negative cases, and produce definitive verdicts.