From cef5a5611a3792cf9eaab5826f48e92c7c287f0a Mon Sep 17 00:00:00 2001
From: Kevin Zhang <kevz8@fb.com>
Date: Tue, 17 Mar 2026 13:08:14 -0700
Subject: [PATCH] Fix unintended high memory usage

---
 lib/compress/zstd_ldm.c |  6 +++++-
 tests/fuzzer.c          | 31 +++++++++++++++++++++++++++++++
 2 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/lib/compress/zstd_ldm.c b/lib/compress/zstd_ldm.c
index 070551cad..7bf36cd86 100644
--- a/lib/compress/zstd_ldm.c
+++ b/lib/compress/zstd_ldm.c
@@ -152,7 +152,11 @@ void ZSTD_ldm_adjustParameters(ldmParams_t* params,
         }
     }
     if (params->hashLog == 0) {
-        params->hashLog = BOUNDED(ZSTD_HASHLOG_MIN, params->windowLog - params->hashRateLog, ZSTD_HASHLOG_MAX);
+        if (params->windowLog <= params->hashRateLog) {
+            params->hashLog = ZSTD_HASHLOG_MIN;
+        } else {
+            params->hashLog = BOUNDED(ZSTD_HASHLOG_MIN, params->windowLog - params->hashRateLog, ZSTD_HASHLOG_MAX);
+        }
     }
     if (params->minMatchLength == 0) {
         params->minMatchLength = LDM_MIN_MATCH_LENGTH;
diff --git a/tests/fuzzer.c b/tests/fuzzer.c
index 472f8c8b3..7b7c9d666 100644
--- a/tests/fuzzer.c
+++ b/tests/fuzzer.c
@@ -1414,6 +1414,37 @@ static int basicUnitTests(U32 const seed, double compressibility)
     }
     DISPLAYLEVEL(3, "OK \n");
 
+    DISPLAYLEVEL(3, "test%3i : ldm hashRateLog > windowLog underflow check : ", testNb++);
+    {
+        /* Test that when windowLog < hashRateLog, we don't get excessive memory usage
+         * due to underflow in hashLog calculation (windowLog - hashRateLog). */
+        ZSTD_CCtx* const cctx = ZSTD_createCCtx();
+
+        size_t const size = (1U << 10); // 1 KB
+        size_t const dstCapacity = ZSTD_compressBound(size);
+        void* src = (void*)malloc(size);
+        void* dst = (void*)malloc(dstCapacity);
+
+        RDG_genBuffer(src, size, 0.5, 0.5, seed);
+
+        CHECK_Z(ZSTD_CCtx_setParameter(cctx, ZSTD_c_enableLongDistanceMatching, ZSTD_ps_enable));
+        CHECK_Z(ZSTD_CCtx_setParameter(cctx, ZSTD_c_ldmHashLog, 0)); 
+        CHECK_Z(ZSTD_CCtx_setParameter(cctx, ZSTD_c_windowLog, 12));
+        CHECK_Z(ZSTD_CCtx_setParameter(cctx, ZSTD_c_ldmHashRateLog, 13));
+
+        CHECK_Z(ZSTD_compress2(cctx, dst, dstCapacity, src, size));
+
+        {   size_t const cctxSize = ZSTD_sizeof_CCtx(cctx);
+            DISPLAYLEVEL(3, "CCtx size: %u bytes ", (unsigned)cctxSize);
+            CHECK_LT(cctxSize, 50 MB);
+        }
+
+        ZSTD_freeCCtx(cctx);
+        free(src);
+        free(dst);
+    }
+    DISPLAYLEVEL(3, "OK \n");
+
     DISPLAYLEVEL(3, "test%3i : testing dict compression with enableLdm and forceMaxWindow : ", testNb++);
     {
         ZSTD_CCtx* const cctx = ZSTD_createCCtx();
-- 
2.47.3