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Max `pBuffer` size for `stse_data_storage_update_data_zone()` unclear due to hidden overhead bytes

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难度
3/5
预计耗时
1-2 天
新手友好度
45/100
Issue 类型
缺陷
描述清晰度
基本清楚
活跃度
冷清
技术栈
c
领域
embedded-iot, security

调研方向

Start in api/stse_data_storage.h, then trace stsafea_update_data_zone() in services/stsafea/stsafea_data_partition.c through stsafea_frame_transmit() in stsafea_frame_transfer.c and stse_platform_i2c_send_start(). Verify how payload, command fields, CRC, and response length contribute to frame_length; done means the maximum pBuffer size and the >= check are clearly established.

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描述

I need to write a 639-byte DER certificate into the data partition (NVM zone index 1) of the STSAFE-A110, which has 700 bytes available. I cannot write all 639 bytes in a single call because we configured our Platform Abstraction Layer to use a buffer of STSAFEA_MAX_FRAME_LENGTH_A110 + 3 (507 + 3 = 510) bytes for I2C communication. So I considered implementing a function that writes the certificate by sending it in chunks of up to STSAFEA_MAX_FRAME_LENGTH_A110 (507) bytes via multiple calls to stse_data_storage_update_data_zone().

The problem is that if I try to write a chunk of 507 bytes, STSE_SERVICE_FRAME_SIZE_ERROR is immediately returned because the following condition evaluates to true (507 >= 507). I’m wondering why >= is used instead of simply >, is that a BUG?:
https://github.com/STMicroelectronics/STSELib/blob/7b34b7c9a5cc35eab6bd7c08c29a41efe3d88d8c/services/stsafea/stsafea_data_partition.c#L381-L383

In any case, I reduced the maximum chunk size to STSAFEA_MAX_FRAME_LENGTH_A110 - 1 (506), but then the failure occurs later due to stse_platform_i2c_send_start(), which returns STSE_PLATFORM_BUFFER_ERR, because frame_length is 513 bytes:

stse_ReturnCode_t stse_platform_i2c_send_start(PLAT_UI8 bus_id, PLAT_UI8 dev_addr, PLAT_UI16 speed,
                                               PLAT_UI16 frame_length)
{
    /* Check buffer overflow */
    if (esc_stse_i2c[bus_id].tx_rx_buffer_length < frame_length) // 510 < 513
    {
        return STSE_PLATFORM_BUFFER_ERR;
    }

    esc_stse_i2c[bus_id].tx_rx_frame_length = frame_length;
    esc_stse_i2c[bus_id].tx_rx_frame_offset = 0;

    return STSE_OK;
}

Note that esc_stse_i2c[bus_id].tx_rx_buffer_length is set by us to STSAFEA_MAX_FRAME_LENGTH_A110 + 3 (507 + 3 = 510).

I would have expected the frame_length argument to be 509 (506 + 3), since here (https://github.com/STMicroelectronics/STSELib/issues/53#issuecomment-3809960369) @TofMassilia13320 mentioned that setting the I2C platform buffer to the maximum value STSAFEA_MAX_FRAME_LENGTH_Axxx + 3 bytes ensures full command coverage of the selected STSAFE-A without overflow at the I2C platform layer.
This led me to assume that STSELib would invoke stse_platform_i2c_send_start() with a frame_length argument that could reach at most STSAFEA_MAX_FRAME_LENGTH_Axxx + 3 (510 in my case).
In other words, I understood that STSELib adds at most 3 bytes to the user data (2 bytes for the response length + 1 byte for the command or response header), as mentioned by @nils-cercariolo-st here: https://github.com/STMicroelectronics/STSELib/issues/53#issuecomment-3802661164. However, this does not seem to hold true, as in my case frame_length is actually 513.

Digging into STSELib, I concluded that the extra 7 bytes are added by stsafea_update_data_zone() and stsafea_frame_transmit(). Specifically:

So the problem is: how can I determine the maximum length of the buffer pBuffer that I am allowed to pass to stse_data_storage_update_data_zone()?
https://github.com/STMicroelectronics/STSELib/blob/7b34b7c9a5cc35eab6bd7c08c29a41efe3d88d8c/api/stse_data_storage.h#L88-L112

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