[Bug] Fix integer overflow in layernorm_kernels.cu pointer arithmetic

[dparikh79]

Summary

Companion to #42861 for csrc/layernorm_kernels.cu. The per-token pointer offsets in three kernels in this file were computed in 32-bit arithmetic when the stride / hidden-size operand was int, overflowing once the product exceeded INT_MAX (about 2.15 billion).

From #42862, the reporter's failing case (model royokong/e5-v, hidden_size = 4096, seq_len = 8129, batch_size = 129) has flat token dimension 1048641, and blockIdx.x * hidden_size = 1048641 * 4096 = 4.29 billion, crossing the 32-bit boundary at row ~524288.

Affected sites (all in csrc/layernorm_kernels.cu)

Kernel	Line	Buggy expression
rms_norm_kernel	69	out + blockIdx.x * hidden_size (the reporter's exact pointer)
fused_add_rms_norm_kernel (vec specialization)	118, 136	int id = blockIdx.x * vec_hidden_size + idx used to index residual_v[id]
fused_add_rms_norm_kernel (generic)	168, 171, 184	residual[blockIdx.x * hidden_size + idx]

Sites already safe (left unchanged)

• rms_norm_kernel line 29: blockIdx.x * input_stride_d2 where input_stride_d2 is int64_t, which promotes the multiply.
• fused_add_rms_norm_kernel (vec) lines 119, 137: blockIdx.x * vec_input_stride where vec_input_stride is int64_t.
• fused_add_rms_norm_kernel (generic) lines 167, 186: same input_stride int64_t pattern.
• rms_norm_kernel lines 32 - 41: division and modulo of blockIdx.x for batch_idx / head_idx / seq_idx, no multiply, no overflow concern.

Fix

Same pattern as #42861 and the existing swigluoai_and_mul_kernel in activation_kernels.cu:

const int64_t token_idx = blockIdx.x;
// ... use token_idx in place of blockIdx.x in the affected multiplications

In the fused_add_rms_norm vec kernel I also widened the local id variable from int to int64_t so it can index the same large flat arrays without truncation when used in the subsequent residual_v[id] reads / writes.

Test plan

• Static review: every blockIdx.x * <int_operand> pointer-arithmetic site in csrc/layernorm_kernels.cu is now backed by int64_t token_idx. Sites with int64_t stride operands are unchanged and remain safe.
• On-GPU repro from [Bug]: integer overflow in layernorm_kernels.cu #42862 (model royokong/e5-v, hidden_size=4096, seq=8129, batch=129): cannot run locally (no equivalent GPU); maintainers with the failing config can verify.

Out of scope (flagged here so they are not lost)

The same class of 32-bit-multiply pattern exists in:

• csrc/fused_qknorm_rope_kernel.cu lines 150, 363
• csrc/fused_deepseek_v4_qnorm_rope_kv_insert_kernel.cu line 157

These are warp-index calculations (blockIdx.x * warpsPerBlock + warpId) of a slightly different shape and should be triaged separately. Happy to follow up in a third PR if maintainers want them addressed.

Sibling of #42861 (activation_kernels.cu int overflow). Fixes #42862.

---

AI assistance disclosure

This PR was prepared with the assistance of an AI coding tool (Claude). The bug diagnosis, the per-site classification into buggy vs already-safe, the int64_t-promotion pattern (matched to #42861 and the existing swigluoai_and_mul_kernel), and the cross-file scan for the out-of-scope follow-up sites were each reviewed by me, and I am responsible for the contents.

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[gemini-code-assist]

Code Review

This pull request addresses potential 32-bit integer overflow issues in the rms_norm_kernel and fused_add_rms_norm_kernel functions within csrc/layernorm_kernels.cu. By promoting blockIdx.x to int64_t before multiplying it with hidden_size or vec_hidden_size, the code ensures that index calculations are performed using 64-bit arithmetic, preventing overflows when processing large tensors. I have no feedback to provide as there were no review comments.

[mgoin]

Thanks for the patch, could you please remove the excessive comment? One line at most imo @dparikh79

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @dparikh79.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[dparikh79]

Going to close this and reopen against csrc/libtorch_stable/layernorm_kernels.cu. #43209 moved the file after your review, so a force-push here would replace the whole diff anyway. Sibling #42861 has the same setup (file moved in #42663). @mgoin happy to force-push in place if you'd rather keep this thread.

[dparikh79]

Closing for #44027 (same fix at the post-#43209 path, comment-trim pre-applied). Thanks @mgoin.