[Bugfix] Fix integer overflow in libtorch_stable/layernorm_kernels.cu pointer arithmetic#44027
[Bugfix] Fix integer overflow in libtorch_stable/layernorm_kernels.cu pointer arithmetic#44027dparikh79 wants to merge 1 commit into
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… pointer arithmetic After vllm-project#43209 migrated layernorm_kernels.cu into csrc/libtorch_stable/, the int32 overflow at blockIdx.x * hidden_size / vec_hidden_size carried over unchanged. blockIdx.x is unsigned int and hidden_size is int, so the product is evaluated in 32-bit and overflows once it exceeds INT_MAX. Reporter's failing case in vllm-project#42862: model royokong/e5-v, hidden_size 4096, seq_len 8129, batch_size 129. The flat token dimension is 1048641, and blockIdx.x * hidden_size = 1048641 * 4096 = 4.29 billion, crossing the 32-bit boundary at row ~524288. Affected sites in csrc/libtorch_stable/layernorm_kernels.cu: - rms_norm_kernel line 70 (out + blockIdx.x * hidden_size) - fused_add_rms_norm_kernel (vec specialization) lines 119, 137 (int id = blockIdx.x * vec_hidden_size + idx) - fused_add_rms_norm_kernel (generic) lines 168, 169, 172, 185, 187 (input/residual indexed by blockIdx.x * hidden_size or input_stride) Sites left unchanged (already safe): - rms_norm_kernel line 30 (input_stride_d2 is int64_t, promotes the multiply) - fused_add_rms_norm_kernel vec line 120 + 138 (vec_input_stride is int64_t, promotes the multiply) - rms_norm_kernel lines 33-40 (division/modulo of blockIdx.x, no multiply) Pattern adopted: const int64_t token_idx = blockIdx.x near the top of each affected kernel, then substitute in the buggy multiplications. Matches the fix shape of the sibling activation_kernels.cu PR and the existing swigluoai_and_mul_kernel pattern in csrc/libtorch_stable/ activation_kernels.cu. In the fused_add_rms_norm vec kernel the local id was also widened from int to int64_t so it can index the same large flat arrays without truncation when used in residual_v[id] reads/writes. A one-line comment at the first site documents the rationale; the subsequent sites use the same pattern without restating it. Reported by @molly-ting in vllm-project#42862 with the exact failing inputs above. Closes vllm-project#42862 Signed-off-by: Dhruvil <dhruvilparikh79@gmail.com>
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What does this PR do?
rms_norm_kerneland bothfused_add_rms_norm_kernelspecializations incsrc/libtorch_stable/layernorm_kernels.cucompute pointer offsets viablockIdx.x * hidden_size(orvec_hidden_size/input_stride) whereblockIdx.xisunsigned intandhidden_sizeisint, so the product is evaluated in 32-bit and overflows once it exceeds INT_MAX.Reporter's failing case in #42862: model
royokong/e5-v,hidden_size=4096,seq_len=8129,batch_size=129. Flat token dimension is1048641, andblockIdx.x * hidden_size = 1048641 * 4096 = 4.29 billion, crossing the 32-bit boundary at row ~524288.Lift
const int64_t token_idx = blockIdx.x;near the top of each affected kernel and substitute into the buggy multiplications. Theint idlocal in the vec specialization is also widened toint64_tso it can index the same large flat arrays without truncation. Matches the sibling fix in #44026 and the existingswigluoai_and_mul_kernelpattern.Sites left unchanged because
input_stride_d2/vec_input_stride/input_strideare alreadyint64_t, which promotes the multiply:rms_norm_kernelline 30, vec lines 120/138, generic line 167/186. The blockIdx.x division/modulo at lines 33-40 has no multiply, no overflow concern.Replaces #42863 (against the pre-migration
csrc/layernorm_kernels.cu) which was opened before #43209 moved the kernels intocsrc/libtorch_stable/. Substantive fix is unchanged. Diff lands at the new path with @mgoin's "one line at most" comment-trim ask pre-applied.Reported by @molly-ting.
Closes #42862
Test Plan
Duplicate-work check
gh pr list --repo vllm-project/vllm --state open --search "libtorch_stable layernorm_kernels"returns nothing else for #42862. Pre-migration sibling #42863 is being closed in favor of this PR.AI Assistance Disclosure
Drafted with Claude assistance. I am the human contributor accountable for this PR; I read every changed line, traced which
blockIdx.x * <stride>sites were already int64_t-safe (and so left unchanged), and verified the int64_t promotion matches the existingswigluoai_and_mul_kernelprecedent.